The Stage as a Character

Automatic Creation of Acts of God for Dramatic Effect


Bradley Rhodes and Pattie Maes
MIT Media Lab
20 Ames St.
Cambridge, MA  02139
phone: 617-253-9601
fax: 617-253-6215
(rhodes|pattie)@media.mit.edu

Presented at the AAAI '95 Spring Symposium on Interactive Story Systems: Plot and Character

The Problem

Picture the climax of an interactive mystery story, wherein our heroine detective explains how she discovered who the real murderer was because of his one careless mistake: he dropped his cigarette butt during his getaway. Such chance happenings are prevalent in all forms of fiction, but making them come about in interactive fiction can sometimes be a challenge. If our interactive story system only allows one player who controls our heroine, we can simply kludge it such that the villain leaves his cigarette butt on his way out the door. But what if, instead of a single player we had ten, each of whom controlled a different main character, including the villain? It is the problem of maintaining plot control with possibly multiple players that we are trying to address.

Our Model of a Story

In our model, a story emerges from the interaction between discrete, autonomous characters, controlled either by humans or artificial systems. Each character has its own beliefs, motivations, and abilities. For example, in implementing The Three Little Pigs, the three pigs have the ability to build houses, party, flee from wolves, etc. They would all have the motivation to have shelter and to have fun, but Fiddler and Fifer Pig (the lazy ones) have a much higher desire to have fun than does Practical Pig. Assuming they are all non-player controlled characters, the pigs choose among their possible actions those that most fit their beliefs and motivations at the time. This means no central system can directly control the actions of a non-player character (which is obviously also the case for player characters). The advantage to such a limitation, at least for research purposes, is that at any time any or all characters in the story can be taken over by players, and the system will largely be unaffected.

Central Plot Control With Uncontrollable Characters

To achieve plot control under these constraints we are designing an extra entity for our story called the stage-manager. Like the non-player characters in the story, the stage-manager is an autonomous agent with abilities and motivations, but it has no body of its own to control. Instead, it controls the physics, chance happenings, introduction of new characters, and "acts of God" which happen in the story. Actions in the stage-manager's repertoire might include "Introduce a new character in the story", "Make the wind blow", and "Make a house fall down." The stage-manager also has its own motivations, which are high-level goals to create certain plot effects. For example, at the start of the story the stage-manager's primary motivation might be to create tension and frustrate Fiddler and Fifer in their labors. This motivation for creating tension drives the stage-manager to introduce The Big Bad Wolf to the story. Later, when the wolf does his huffing and puffing thing, it is the stage-manager who decides whether the houses fall down or not. This decision is not based on a carte-blanche decree that straw and twig houses can be blown over and brick houses can't, nor is it based on some underlying physics model. Instead, the stage-manager decides the fate of the house based on it's own desires for how the plot should progress.

The autonomous characters in our story are constrained by the physics of our world: the wolf cannot walk through the brick walls, the pigs cannot fly. The stage-manager, on the other hand, determines the physics of our world, and is theoretically omnipotent. However, the stage-manager must limit itself by artistic constraints: to create a believable story-line there must be an internal consistency in the universe. Some events (actions of the stage-manager) can simply occur out of nowhere and be perfectly believable; a candle can go out, the wind can start to blow, or a character can enter the scene without needing any setup for the audience to find it acceptable. Other actions need no explanation, but seem unlikely. For example, a freak meteor shower or an earthquake are both unlikely but possible without some other cause. Finally, some events naturally draw the audience to look for a cause. If the brick house fell when no one was around it and no wind was blowing, the audience would naturally want to discover a reason and would be upset if they didn't. Just as the pigs are strongly motivated to escape from the wolf, even at the expense of their motivation to play, the stage-manager must be motivated to maintain believability even at the expense of other plot-relevant goals. It is this motivation for believability that stops the stage-manager from satisfying its desire to thwart Fiddler and Fifer by having a meteorite fall on their houses or other such direct routes. The introduction of the wolf, while less direct in thwarting the pigs, is much more believable.

The Relativity of Believability

The need for internal consistency does not necessarily imply that everything happening in the world is realistic, or plausible in the real world. Rather, it implies the much weaker requirement that everything the audience and player(s) see in the world is internally consistent. For one, this allows the story director to "cheat" and make up anything that happens outside of the audience's view. For example, the stage-manager needn't have known that The Big-Bad Wolf was even in the story until it decided to introduce the character somewhere near the straw house. Soap operas often use this technique: if a major character dies in a shipwreck and enough viewers complain, six months later it is suddenly "discovered" that he had been saved at the last minute and had been living on a deserted island. Note that this technique is more effective when the audience sees only a limited field of a much larger world, or when there are a small number of people observing parts of the world. If there are large numbers of people watching a complicated story unfold from different vantage points (for example, in a multi-player MUD or MOO where players roam throughout the world), less cheats can be found which are not inconsistent with at least one of those people's previous views.

Consistency also depends on the type of story being conveyed. There is no violation of consistency when Bugs Bunny produces a disguise out of nowhere or makes a safe fall on another character's head, because that's a part of "'Toon Physics." Similarly, even though the coincidences in a Gilbert and Sullivan opera are improbable to the point of being ludicrous, these coincidences are consistent with the genre that is established at the very start of the play.

Current Research

In our current research, we are implementing the story of The Three Little Pigs using extensions of behavior networks [Maes, 1989] for both the stage-manager and characters. In behavior networks, agents are made up of competence modules (black boxes representing possible actions that can be taken by the agent) interconnected in such a way that "appropriate" actions are taken even in a rapidly changing and not completely predictable world. Some example competence modules for the pigs are "build house," "run away from wolf," and "dance and sing." Some example competence modules for the stage-manager agent are "make the ground beneath the wolf's feet slippery," "introduce the wolf as a character," "make the straw house fall down," and "make the wolf die of a heart attack." Which action to take next is determined by a process whereby activation energy originating from the motivations and sensor data of the agent is spread to relevant modules.

The system is being designed with the intent that humans will be able to take the place of any of the characters in the story, and possibly even the stage-manager itself. However, our initial research focuses on only allowing a player the role of a "plot director." In this role, the player crafts the story by selecting the initial state of the world and the various motivations of the stage-manager throughout the story.

For example, if the user wishes to tell The Three Pigs in its original form, she would start with the three pigs "on stage" and at first not give the stage-manager any motivations except to be believable. The pigs would proceed to make their straw, twig, and brick houses as befit their motivations. After all three houses have been built, the player would give the stage-manager the motivation to create tension. Assuming there are no equally believable options, the stage-manager would automatically introduce The Big-Bad Wolf near the straw house to achieve this goal. The wolf would then go to the straw house and, in attempting to achieve it's own goal of eating pigs, would try to blow the house down. Because having the house fall down would create tension, and because having the house fall over is believable in the current situation, the stage-manager would make the house fall over. However, as the wolf chased the pig, the stage-manager would perform some action to insure that the wolf doesn't catch and eat the pig, as eating the pig would be a resolution of the tension it wants to maintain. This action might be as subtle as making the wolf slightly slower than the pig or as obvious as making the ground beneath the wolf's feet slippery so he trips a lot.

After the two pigs have had their houses blown down and they run to the brick house, the user would change the stage-manager's motivation to resolve the conflict and thwart the wolf. Now when the wolf huffs and puffs, the house wouldn't fall. Furthermore, in order to resolve the conflict, the wolf might die of a heart attack, presumably from the exertion of all that huffing and puffing. As the pigs follow their natural motivations to dance and play, the stage-manager can close the curtain with the words "The End" flowing across the scene.

Different motivations for the stage-manager would produce a very different unfolding of the story. For example, say the stage-manager still has a high motivation to be believable, but after the three houses are built the player gives it the motivation to screw over all three pigs. The stage-manager might still introduce the Big Bad Wolf, but this time when the wolf blows down the straw house, it will be the pig who trips and stumbles. The wolf will catch and eat the pig, and continue on to blow down both the twig and brick houses and eat the other two pigs.

As a final example, say after the pigs make their respective houses the player gives the stage-manager the motivation to screw over all three pigs, but also reduces the motivation to be believable. The stage-manager could still introduce the wolf to achieve this goal, but without the constraint of maintaining believability it might very well simply drop safes on all three pigs. Curtain.

We are currently implementing the above system, and plan to show a working model in which a user can change the motivations for the stage-manager and see the changes in the resulting story-line.

References

Maes, Pattie. (1990) How to do the Right Thing. Connection Science, Vol. 1, No. 3, 1989

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