We are interested in the application of interactive storytelling to videogames. We want to improve story experiences in open-world adventure and role-playing games. A game that features an open world allows its players to move freely in a large space with few or no artificial barriers, choosing what to do and when. The flexibility of an open world and the fact that adventure and role-playing games tend to have strong story components make these genres an interesting place to explore interactive storytelling techniques.
Our central goal is to support the creation of open-world videogame stories that give players a sense of coherence. To achieve this, we take a structuralist approach and partition stories into two types of scenes inspired by the concept of kernels and satellites. First, a minimal set of fixed scenes form a core story with strong authorial control. A game’s most central plot points become fixed scenes, thus acting like kernels. The rest of the story emerges from a much larger collection of flexible scenes that can appear just about anywhere in story save a small set of preconditions. Most flexible scenes act like satellites: minor plot points, or opportunities to develop story elements like theme.
We want to give players the freedom to explore flexible scenes however they wish as they move through the fixed scenes as designed. A certain level of coherence is guaranteed when the content of the fixed scenes is itself coherent, but a story with few satellite scenes will have minimal aesthetic appeal. The challenge, then, is to maintain coherence no matter how a very large set of flexible scenes is experienced.
Instead of arranging flexible scenes according to a strict definition of causal coherence, we want to create a “sense of” coherence. By this we mean that not all events have to be causally related in explicitly obvious ways, but that players should have the sense that they could figure out the meaning of and relationships between events if they thought hard enough about it.
One of the major ways we achieve a sense of coherence is by managing the story’s progression. We keep track of when certain story elements, such as theme and character, are reflected. We then prioritize which scenes should be made available to players next according to a desired distribution of the story elements. For example, if a particular theme was developed very recently, we want to prioritize scenes that reflect some of the other themes. On the other hand, if it has been a long time since a theme was developed, scenes that reflect that theme strongly should have high priority. A good distribution of elements ensures that story elements don’t feel out of place when developed, and that reminders of previous scenes are made throughout the story.
Another facet of creating a sense of coherence is the emergence of structure at run-time through the use of conditions. Instead of defining causal relationships in a scene graph a priori, we allow authors to define prerequisites for their scenes. Using prerequisites is a common technique, but in our design we push for prerequisites based on story state values in addition to game state. For example, scenes might have prerequisites that only allow them to be seen once a particular theme has been developed sufficiently. Alternatively, a scene might be best suited for the early development of the theme, and should not appear later on. We want authors to think about flexible scenes in terms of how they function in a story’s development without having to worry about how they will fit within a series of causally related events.
In addition to controlling the path players take through a set of fixed and flexible scenes, we can improve the sense of coherence by adjusting the content of scenes. In so doing, we want to give players interpretative agency: they should feel like there are deeper layers in the story not being explicitly told, and they should feel like they can interpret those layers in a reasonable way.
We are exploring three ways of dynamically affecting the content of scenes. In the first, run-time criteria is used to choose a set of scenes that a recurring motif (say, an apple) can be featured in. Observant players will begin to notice the motif over time and assign meaning to why it appears in certain scenes. Eventually, they will expect something in particular to happen when a new scene with the motif begins.
Second, mix-ins give us pre-scripted opportunities to make connections to scenes the player happens to have already seen. As Keith Johnstone points out in the context of improvisation, “feeding something back in from earlier in the story adds ‘point’ and creates structure.” Characters, story elements, and dialog are all examples of source material that could be referred to in future mix-ins.
Finally, we can adjust the presentation of a scene to alter the player’s interpretation of otherwise unchanging events. Choice of lighting, background music, camera angles, and even the weather can all depend on the story’s state at the time a particular scene is reached. Perhaps the heroine of the story returns to the castle with the head of a dragon. The mood evoked during the scene might be bright and cheerful if the player saw the dragon as an evil menace. However, the mood might be more sombre if the player found out that the dragon was simply a loving mother trying to protect her hatchlings. The final event stays the same, but the interpretation of it changes.
In summary, our goal is to give players a sense of coherence when exploring stories in open-world adventure and role-playing games. We structure our stories as a set of fixed and flexible scenes. Players can traverse the set of flexible scenes freely, barring any prerequisites that deem certain scenes inaccessible. Flexible scenes are prioritized so that story elements are well distributed throughout the story. We encourage interpretative agency by dynamically introducing recurring motifs, using mix-ins to make connections to earlier points in the story, and modifying the presentation of a scene to affect interpretation. Through all of this, higher quality open-world stories will emerge while still maintaining a satisfactory level of interactivity.
