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