The Interaction Engine

The Interaction Engine

I am lying on the big double bed in the middle of the living room, all clad in white. A glass of whisky in hand and Ray-Ban Aviators on to conceal where I am looking. The memorial for Lena who committed suicide earlier that year, is ongoing. Her widower Wilhelm is giving a tear-ridden speech about how wonderful a wife and mother Lena was.

House-Wilhelm, a ghost-character portraying both the House they are in and a previous incarnation of Wilhelm, is forcing the fingers of his son down his own throat, trying to throw up as a comment to the speech given by his human counterpart. In between, he screams that Lena was never supportive of him and that he hopes she burns in hell. None of the human characters react to this. They can not see or hear the twelve House-ghosts that are in the room with them.

The ghosts, on the other hand, applaud Ghost-Wilhelm’s effort to throw up and laugh scornfully when he fails. He falls sobbing to the floor in front of his human counterpart while repeating “I am sorry, Lena!” over and over.

The human characters step over him and go to get coffee and cake in the adjoining room. This scene will repeat itself in new and equally exciting ways for the next seven days when the human characters will be absorbed into the house by midnight and become the House-ghosts for the new family that will come to stay in the house.

What I[1]Although the article has two authors, it is written from Bjarke Pedersen’s perspective. just described is a scene from the erotic horror larp House of Craving (Denmark 2019). I was there playing one of the House-ghosts as a non-player character. The larp was created by Danny Meyer Wilson, Tor Kjetil Edland, Frida Sofie Jansen and myself. Eighteen runs have been played since the premiere, and the original freeform (Længslernes Hus, 2017) has run countless times since its premiere at Fastaval in Denmark. The larp runs for eight days straight, with twelve new participants arriving each day, reiterating the story lived by the characters. On the first day, you play as the family who has come to spend time at a haunted house. Around midnight, your character is absorbed into the House, and the next day, you play a House-ghost version of your character.

Each run is uniquely different but feels much the same. The structure of the larp is based around a few repeating fate play scenes each day that gather people together. One of them is a memorial service for Lena, the mother who has committed suicide. The fate play scenes and the ending for both the Family and the House characters are predetermined. Everything else is created by the participants on the fly, together as an ensemble. There are no quests and little connected narrative or story per se. The participants are encouraged to prepare as little as possible besides reading the larp materials and getting their costumes. The larp becomes better without preparation. With less preparation, there will be fewer assumptions about what the larp will be, and the participants are better equipped to react in the moment.

This works because the larp is designed around what I call an interaction engine. The interaction engine is a specific type of larp design where the primary focus is on enhancing playability by ensuring that every action generates new possibilities and emotional impact. Other larp design styles may foreground structured narratives, fighting simulations, or realism, for example. These can exist in engine-driven games, but they are always in the background. The main focus in an engine larp is on what creates interactions between participants. Specifically, interactions that intensify the larp experience – the aim is not to create intensity for the sake of it, but intensity that moves both the individual and the ensemble experience in the direction of the themes of the larp. This way, the journey through the experience by the player will be way more dynamic than a plot or narrative written months before the participants arrive at the location. Scenes and experiences that players create themselves on the fly will fit better into their context and into what they want to experience.

The interaction engine will help the players create engaging interactions that are both emotionally and physically intense, and that always lead to more interactions rather than fewer. The goal is to get the players to connect with the themes of the larp in as many ways as possible, so their actions resonate with not only their own dreams and desires but also with their cultural identity, experiences in their own lives, and how they see themselves.

I came to this design method out of frustration. Early on in my design career, I realized that a big part of the work that my team or I had done was never used. If it was a plot or story, then the players went in another direction than I anticipated. Or if it was character relations, then the chemistry of the players was off, or the relationship was not something that was interesting for the people involved. All the hard work creating what I felt was good content for the larp, was wasted time I could have used on other aspects of the larp.

The most scarce resource you have in larp is the organizers’ time, closely followed by the participants’ time. When a large part of the material created for the larp is not used, you have wasted both. What I realized was that to create the best possible larp and not waste time, you need to let go of the narrative control and hand it over to the participants. Instead of controlling their narrative journey in great detail, an interaction engine guides the players to understand what the themes they are supposed to explore are, and the chosen larp mechanics help them to do that in the best available way. You could say that an interaction engine larp controls the emotional arc of the character’s story, rather than the narrative arc.

When you design a larp, you often start working on the parts that are the most exciting to you. While this can be rewarding and motivating, the design of an Interaction Engine larp needs to begin with laying down a core foundation that must form the basis of all your future decisions. In an engine design approach, you have to start with the theme or themes for your larp. You then map out the actions that your participants can do that support the exploration of those themes from as many angles as you can think of. By actions, I mean very specifically the things that the players actually will be doing during the larp. What verbs describe the actions in the best possible way? For House of Craving, some of the verbs are flirting, controlling others, lounging, and masturbating.

These actions define the focus of your larp. All other design choices should be made to support them. If you want flirting, then flirting needs to be front and centre in the workshop, to be sure people trust each other and are comfortable with each other – and even more trust and comfort are needed when it comes to masturbation. For House of Craving, we had a masturbation mechanic that included clear jelly dildos as penis replacements, and we workshopped it extensively before the larp. The ghost-penises as they were fondly called by the players, could be used by the players no matter their character’s gender. We also instructed the players to always bring a ghost-character with them if they went to their room to masturbate. The actions we wanted to see were supported by the mechanics of the larp and the workshop design, helping the players play with otherwise private and intimate actions that are very difficult to do without support from the design.

There are larps with themes where an interaction engine is probably not the best fit. You need the themes to be focused on emotional and relational actions for the interaction engine to truly shine. An example of a larp where an interaction engine would be less ideal could be a larp centred around detail-oriented and rules-heavy diplomacy, where the actions have to follow a predetermined structure to achieve connection to the themes.

Building a larp around an engine demands that all elements, from scenography and food to characters, relations, and motivations, are aligned toward the themes and actions of the larp. Everything else should be removed from the design. When done well, this makes the theme of the larp accessible and playable for all. The player does not have to be at the right place at the right time to access the important plot – they can create access for themselves at any given time by being in the setting and engaging with the design and mechanics.

When designing a larp this way, any main plot steps into the background and the potential for meaningful encounters between characters is brought to the foreground. When all the verbs or potential actions available to the players are clearly defined and understood, the players can choose the ones that make the most sense to them at any given time. If the participants understand the themes, and it is clear how the verbs connect to the themes, all the participants are able to steer their experiences in the same direction, each individually choosing the best possible path for themselves.

The larp mechanics allow the participants to push their actions (described by the verbs) beyond what is possible without mechanics. Moreover, the mechanics make it easier and faster for the participants to take the actions described by the verbs. Thus, the mechanics work as tools to create or support powerful moments. Giving participants the responsibility and trust to follow their own desires (through the lens of their characters’ motivations) as to what to explore gives room for their imaginations to shine. Given space, they will tell far more gripping stories than you as a designer can ever create for them.[2]To be clear, gripping and meaningful are not always the same thing engine-driven larps do away with a lot in the pursuit of engagement. That said, in my experience more games have failed because the players have been disengaged than because the deeper meaning of the larp did not resonate deeply enough in the lives of the players.

This, of course, demands a lot from the players. A high level of herd competence is required for the engine to run smoothly. With less experienced players, more workshop time is needed for a smooth larp experience. The extra workshop time should primarily be used to create trust within the ensemble and to help individual players calibrate with the norms of the ensemble. Another use for it is to ensure that the players understand what they are personally comfortable with and what they are interested in experiencing.

Juhana Pettersson writes in his excellent article Engines of Desire:

“When I conceptualize the process of larp design, I see it as working with the players to give them the desires required by the design and help them get in touch with their own desires so they can use them to drive action. When a player does something they’ve always wanted to do, they bring energy and power to the larp. You can see it in the way people play, carry themselves, speak, act. It’s a powerful thing and generates so much meaning.” (Pettersson 2021)

The most important thing for an engine-based larp is to create a space where the participants feel safe and seen and where they feel they have the possibility to explore and engage with the themes of the larp without fear of being ridiculed or having their boundaries breached. In this mind-space, the participants feel empowered, with all possibilities open for them to choose. Many of my participants have told me that being in this space feels both overwhelming and totally safe. In these moments, larp can be transformative. You learn something new about both yourself and the world when you dare to step up to the edge of your safe interaction space and into unknown territory.

That the participant feels like they have all possibilities open to choose from is of course an illusion created by well-crafted larp and participation design. This design starts way before the participants arrive on site.

“Everything is a designable surface” is the mantra for all of my design work. It was coined by Johanna Koljonen (2019), and it means that all the design decisions you make or that are made for you by e.g. time or monetary constraints, a protected historical location, or anything else beyond your control, will have an impact on the success of your larp. For instance, if the temperature in a room is a few degrees too cold the characters will not take their coats off or sit still for very long, and your well-planned physical boudoir interaction space goes out the window – as happened in a 2018 run of Inside Hamlet.

As a designer, you literally need to think of everything – or, more practically, you need to accept that you are responsible for all aspects of the larp even if they are out of your control. At any given moment when designing or running a larp you should ask yourself the question “What are the consequences of making this decision and not another one?”

Use the themes you have set for your larp as a guide. If all of your decisions are aligned to support the themes, you are well on your way to creating an interaction engine larp.

But what is the interaction engine? Can you point at it? Just like a real engine, an interaction engine is made of hundreds of parts (which we don’t have room to describe exhaustively), and no one part can be said to be the whole thing. To start identifying the core of your engine, ask yourself the following question: “What is the main part of the design that drives participants to actions that are connected to the themes of the larp?”

The answer to this question is the core of the engine, and you should put your design effort here to support this part of the design in as many ways as possible. The more time and energy you use here, the easier your design decisions will be.

To be able to answer the question above, you need to analyze the themes of your larp and describe them in detail – an example will follow below. With your themes locked down, you then need to figure out what design elements will most efficiently drive your participants to perform actions that connect directly to those themes. This is the core of your interaction engine.

Once you have the core of the interaction, you need to iterate through all aspects of the design with your themes and the core engine in mind. This means looking at your larp mechanics, your set and spatial design, costume guidelines, your workshop structure, how food is served, the website, participant communication, and everything else. All of these should be focused on supporting the themes and the core engine to drive participants to take actions during the larp that explore the themes in the ways that you think will be most worthwhile. As you make new decisions about different parts of the larp, you need to continually cross-reference with all the other decisions you have made to ensure that you do not make choices in one place that push players toward an action that you have made harder for them in another place.

As you are doing this, you may identify actions that your design pushes participants to do that are not connected to the themes of the larp. In a few specific cases, these might feel necessary to make the larp feel coherent to some players, allowing them to access the rest of the game, but this is rare, and almost without exception you should remove them from your larp. If you do not, these actions will feel disconnected from the rest of the larp and be uninteresting to engage with for your participants and may lead to participants falling out of play or being confused about the things that they should be doing.

For example, if you are making a larp about a decadent court and some of the characters are guards designed to stand still and guard the court, then you will have a group of characters that are not able to engage with the themes of the larp. At Inside Hamlet, we solved this challenge by making the royal guard more like celebrities that the members of the court wanted to become or to bed. These celebrity guards did not need to stand guard at all.

An Example: PAN

PAN (Denmark 2013) is an example of an engine-based larp. A group of couples from various walks of life are at a couple’s therapy workshop retreat run by a new-age husband and wife. Over the course of a weekend, the participants go through various exercises trying to save or improve their relationships. In one of the more new-age exercises, the workshop leader does a seance, trying to connect with people from the other side. This fails spectacularly when the Great God Pan enters our reality and possesses her. Pan then starts to jump from person to person over the next few days until all notions of reality and identity are stripped from the characters and all characters are willingly destroyed.

The themes of PAN are an exploration of self-actualization in a couple structure, what ethics, morality, and being civilized actually mean, and what happens when this is stripped away. What is then left of a person’s humanity? Some of the actions that are connected to the themes are possessing, indulging, taking control, losing control, being shameful, being fearful, exploring the self, and destroying your relationship, among others.

The core of the engine in PAN is the possession mechanic. The Great God Pan is symbolized by a necklace. The necklace is only visible to participants, not their characters. Wearing the necklace, and seeing someone else wearing it, both have specific interaction scripts.

When you are wearing the necklace, you become possessed by Pan and must pursue your biggest basic needs as soon as possible – if you are hungry you must eat and if you are horny you must find release. Pan does not care for what is proper or in good taste.

If you see someone wearing the necklace, your character will ignore everything around themselves, and the possessed person becomes the single most interesting thing in the world. You will do anything at all to get their attention, to have them see you, touch you.

This leads to mayhem. The necklace leaves broken and embarrassed characters in its wake, with each possession adding a new and different layer of emotional chaos to the characters impacted by it. Every possession is unique, driven by what the participant wearing the necklace wants and desires from the larp at that moment. The agency goes both ways, too – if a participant around the possessed doesn’t find their desires in the interactions around the necklace, that participant leaves the room and pursues play somewhere else.

The only planned scenes in the larp are the seance where Pan enters the world and the ending where everyone follows the god into oblivion. All scenes that arise because the necklace travels from participant to participant are unscripted. They evolve and change in each iteration like a beautiful fractal pattern. This way, the participants tell stories that we the designers never could imagine in our wildest dreams.


Creating a larp designed around an interaction engine demands more design work at the beginning of the process, but it pays off later by giving you a guiding light for every decision you make. When you identify the core themes and verbs for your larp it helps you focus on the actions and larp mechanics you should be designing, leading your participants to do engaging and coherent things together.

Finally, this essay includes some of the questions you can ask yourself to help you design an interaction engine larp. As an example, I will in the next section answer some of the relevant questions for my larp PAN. Please add your own questions to the list as you work with this design style:

  • What are the core themes of your larp idea?
  • How would you describe each theme in such a way that every participant will be able to understand it?
  • Why these themes and not other ones?
  • What actions explore the theme? How many different types of actions can be used to do so?
  • Are there any actions currently in the larp design which do not connect to the themes? Can they be removed?
  • How can you support the core actions by planning secondary actions around them?
  • Are the core actions accessible to all characters and participants? If not, why?
  • What affordances in the design, site, mechanics, characters, or costumes are required to make those actions possible and legible to participants?
  • How can you design all aspects of the physical space to support the actions that you want and make them desirable to participants?
  • How can you shape the use of time, either the participants’ time on site or before the larp or the structure of time inside the larp, to support those actions?
  • What communication strategy will best support the interaction engine?

The origins of PAN

The design of PAN began when my co-designer Linda Udby and I were sitting and complaining that there were no larps to sign up to that we were interested in. After some time we ended with a conclusion that I can highly recommend: we decided to make our own damn larp!

I was really interested in exploring the, at the time, new idea that you need an alibi to be able to play a larp that is intense and outside your comfort zone, and that you can design such an alibi. We wanted to make something quick and dirty that would not take a year to design and produce nor require endless preparation from the participants. This restricted what kind of larp we could make in many ways. For example, we needed a location that we could use as it is without having to build or dress.

I had just read the gothic horror story The Great God Pan (Machen 1890) and was fascinated with the idea that there was merely a thin veil protecting us from a reality so alien that seeing it would shatter our morals and beliefs and drive us insane. With these restrictions and ideas, we came up with the core idea for PAN. The larp is set in the present day since this choice made it easier for us to find a location and to produce the larp and easier for the participants to find costumes. We chose a summer house as the location. The number of characters in the larp was decided based on the number of beds at that summer house.

Back in 2012 when we designed PAN, I would have answered the (relevant) design questions from the previous section as follows (as far as I can remember).

What are the core themes of your larp idea?
Exploration of self-actualization in a couple structure; what ethics, morality, and being civilized actually mean, and what happens when this is stripped away.

How would you describe each theme in such a way that every participant will be able to understand it?
The experience of PAN will take your character through working on your relationship in a new age therapy weekend in a group with people you have never met. Suddenly you will be face to face with a god that will slowly strip you of everything you know. You will end up betraying yourself and your partner in the most heinous and terrible ways.

Why these themes and not other ones?
They fit this specific larp very well, they are themes that I am very interested in right now, and they will expand my knowledge of designing and running larps. Moreover, the themes can be explored within the time and production restrictions we have set.

What actions explore the theme? How many different types of actions can be used to do so?
The actions available are grouped into two different categories. The first is a group of actions that are connected to the self-actualisation and therapy part. Here the verbs are going to be: engage with therapy, argue, expose shame and lust, meditate, perform relationships, help others to open up, etc. The second group of actions are the ones that the god forces upon the participants via the game mechanics and instructions on what to do when possessed or seeing someone who is possessed. Here the verbs are indulge, scream in terror, give in to lust, and abuse others and yourself.

How can you support the core actions by planning secondary actions around them?
Since the larp was so small (8–12 players), there was little room for secondary actions. Each couple in the larp had their own story that had some secondary actions embedded. It was not a priority to make this consistent across all characters during the design process (the larp would be better for it, though).

What affordances in the design, site, mechanics, characters, or costumes are required to make those actions possible and legible to participants?
To play PAN, you had to agree to play the larp in a very physical style, and you needed to understand that you are not in control of the character’s journey. Even if the ending of the larp was predetermined, neither the participant nor we the designers were in control of what would happen during the larp. This was due to the chaotic narrative the possession mechanic enforced on the larp.

How can you design all aspects of the physical space to support the actions that you want and make them desirable to participants?
The location needed to be open and small, with few places to hide and be private. We needed to be able to hear where the participants were and where the participant that was currently possessed by Pan was moving.

How can you shape the use of time, either the participants’ time on site or before the larp or the structure of time inside the larp, to support those actions?
The biggest challenge in a larp like PAN is to make the participants feel safe enough to fully engage in the actions the larp is aiming for. This is why we decided to use more time in the workshop to create trust amongst the participants than in many other larps. It also meant a quite harsh casting process. You needed to sign up with the person you would play partner with. We did this to make sure that there would be trust between the players of couples already right from the beginning.. The whole ensemble was chosen based on a signup form where you had to motivate why you wanted to participate.

What communication strategy will best support the interaction engine?
We had a simple website with enough information to understand what the larp was about and what was required from the participants. We deliberately avoided creating hype around the larp, since we wanted to make sure that only people who were truly interested in the themes and actions would sign up.


Arthur Machen (1890): The Great God Pan. Whirlwind magazine.

Johanna Koljonen (2019): An Introduction to Bespoke Larp Design. In Larp Design: Creating Role-Play Experiences, edited by Johanna Koljonen & al. Kopenhagen: Landsforeningen Bifrost (Knudepunkt 2019), p. 25–29.

Juhana Pettersson (2021): Engines of Desire in Engines of Desire: Larp As the Art of Experience, p 247. Pohjoismaisen roolipelaamisen seura.


Inside Hamlet (2015): Denmark. Participation|Design|Agency

Pan (2013): Denmark. Linda Udby & Bjarke Pedersen.

House of Craving (2019): Denmark. Danny Meyer Wilson, Tor Kjetil Edland, Frida Sofie Jansen & Bjarke Pedersen.

This article has been reprinted with permission from the Solmukohta 2024 book. Please cite as:

Pedersen,Bjarke & Eleanor Saitta. 2024. “The Interaction Engine.” In Liminal Encounters: Evolving Discourse in Nordic and Nordic Inspired Larp, edited by Kaisa Kangas, Jonne Arjoranta, and Ruska Kevätkoski. Helsinki, Finland: Ropecon ry.

Cover photo: Photo by Hoover Tung on Unsplash

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1Although the article has two authors, it is written from Bjarke Pedersen’s perspective.
2To be clear, gripping and meaningful are not always the same thing engine-driven larps do away with a lot in the pursuit of engagement. That said, in my experience more games have failed because the players have been disengaged than because the deeper meaning of the larp did not resonate deeply enough in the lives of the players.


Eleanor Saitta (b. 1978) is a hacker, artist, designer, writer, and barbarian making a living and vocation of understanding how complex, transdisciplinary systems and stories fail and redesigning them to fail better.
Bjarke Pedersen is the creative director of Participation Design Agency. His primary work is designing participatory events and experiences with a focus on strong stories. In the last twenty years he has worked on projects ranging from transmedia storytelling for television to real-world participatory games with more than 10.000 people playing at the same time. He is a world-leading expert in the design of bespoke larps for grownups. These immersive experiences include Inside Hamlet at the castle Elsinore in Denmark, and End of the Line, the first official collaborative style larp product from White Wolf, produced in Helsinki, New Orleans and Berlin. Bjarke teaches and lectures on participatory design and culture at universities, conferences and art festivals across Europe and USA. He is a co-founder of the Alibis for Interaction masterclass.