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226 Socially Intelligent Agents

Trip’s line of dialog (attempting to change the topic), and the player’s response
is a beat. Now if the player responds by accepting Trip’s offer for a drink,
the attempt to change the topic was successful, Trip may now feel a closer
bond to the player, Grace may feel frustrated and angry with both Trip and
the player, and the degree to which relationship problems have been revealed
does not increase. On the other hand, if the player directly responds to Grace’s
line, either ignoring Trip, or perhaps chastising Trip for trivializing what Grace
said, then the attempt to change the topic was unsuccessful, Trip’s affiliation
with the player may decrease and Grace’s increase, and the degree to which
relationship problems have been revealed increases. Before the player reacts
to Grace and Trip, the drama manager does not know which beat will actually
occur. While this polymorphic beat is executing, it is labelled "open." Once the
player "closes" the beat by responding, the drama manager can now update the
story history (a specific beat has now occurred) and the rest of the story state
(dramatic values, etc.).

4.4 Joint Plans
Associated with each beat is a joint plan that guides the character behavior
during that beat. Instead of directly initiating an existing goal or behavior within
the character, the drama manager hands the characters new plans (behaviors)
to be carried out during this beat. These joint plans describe the coordinated
activity required of all the characters in order to carry out the beat. Multi-agent
coordination frameworks such as joint intentions theory ([15]) or shared plans
([3] provide a systematic analysis of all the synchronization issues that arise
when agents jointly carry out plans. Tambe ([17]) has built an agent architecture
providing direct support for joint plans. His architecture uses the more formal
analyses of joint intentions and shared plans theory to provide the communi-
cation requirements for maintaining coordination. We propose modifying the
reactive planning language Hap ([11]; [10]), a language specifically designed
for the authoring of believable agents, to include this coordination framework.
Beats will hand the characters joint plans to carry out which have been
designed to accomplish the beat. This means that most (perhaps all) of the high
level goals and plans that drive a character will no longer be located within
the character at all, but rather will be parcelled out among the beats. Given
that the purpose of character activity within a story world is to create dramatic
action, this is an appropriate way of distributing the characters’ behavior. The
character behavior is now organized around the dramatic functions that the
behavior serves, rather than organized around a conception of the character
as independent of the dramatic action. Since the joint plans associated with
beats are still reactive plans, there is no loss of character reactivity to a rapidly
changing environment. Low-level goals and behaviors (e.g. locomotion, ways