PaSSAGE (Player-Specific Stories via Automatically Generated Events) is an interactive storytelling system whose primary goal is to take advantage of the wealth of feedback that the audience of an interactive story provides [1,2]. By automatically learning the preferences of its audience, PaSSAGE aims to maximize the quality of its stories on a person by person basis, providing each player with the particular sequence of events that he or she will enjoy the most.
David Thue created PaSSAGE as his M.Sc. thesis project in 2006, and it is currently the focus of his Ph.D. research. It is implemented in BioWare Corp.’s Aurora Neverwinter Toolset, the software tool used to create the successful computer role-playing game, Neverwinter Nights. Although PaSSAGE is not yet available to the public, interested writers and/or researchers are encouraged to either visit the project website (http://www.playpassage.com), consult our publications (links below), or contact the authors directly.
The primary distinguishing features of PaSSAGE are that it learns about its players while they experience its stories, and it allows authors to use what it learns to inform a wide variety of story-related decisions, through a paradigm called "Delayed Authoring" . By allowing Delayed Authoring, PaSSAGE aims to grant its authors the ability to delay their decisions concerning story content for as long as possible, and particularly past the time at which a given player’s story begins. By waiting to decide about story content until right before the content is needed, authors gain the benefit of having the extra, player-specific information that PaSSAGE has learned during the story so far. In this sense, PaSSAGE is designed to act as a decision-making proxy for its authors; they describe how a decision should be made for different types of players, and then PaSSAGE carries out their decision once a player’s type has been learned. For example, concerning a particular event in the story, PaSSAGE allows its authors to delay the following types of decisions:
- What should happen?
- How should it happen?
- When should it happen?
- Where should it happen?
- Who should be involved?
- Why should actors act as they do?
PaSSAGE includes three mechanisms for making these decisions as a proxy for its authors: Courses of Action, Triggers, and Role Passing.
Courses of Action: Deciding What and How
When a new event is needed for the story (perhaps to comply with an author-specified schedule), PaSSAGE chooses from a library of potential story events, each of which contains one or more courses of action for the player to take. Courses of action describe the different ways in which a player might respond to a particular story event, while that event is taking place. While courses of action are similar to the branches of typical story-tree representations, the main difference between the two is that story-tree branches typically correspond to player action *between* story events, while courses of action correspond to player action *within* story events. For each course of action, authors must describe the characteristics of the type of players who they expect would enjoy taking it more than the others. These descriptions are attached as annotations on each event, and then used by PaSSAGE during the story to select events that its particular current player will enjoy. Once an event begins, each course of action therein becomes available to the player; allowing players to choose between them is crucial to PaSSAGE’s operation, as such player choices are what allow PaSSAGE to learn its players’ preferences. Given that PaSSAGE can estimate which of the available courses of action best suits its current player, it is also able to modify the actions of the event (affecting How it occurs) to steer the player toward taking that course of action. For example, if PaSSAGE learns that a player enjoys combat, then the non-player characters in an event can be made to attempt to start a fight.
Triggers: Deciding When and Where
PaSSAGE’s stories are set in a simulated world, and this allows the occurrence of events to be triggered by various aspects of the current world state. Triggers monitor the world state, repeatedly testing one or more conditions; each condition describes an aspect of the world state that is necessary for the trigger’s associated event to occur. In particular, triggers concerning the location of the player or non-player characters can be used to ensure that events only occur at appropriate times. For example, one might create a trigger to ensure that a robbery-themed event only begins to unfold when the player arrives at the local bank. If the robbery event were authored to support happening in more than one bank, then PaSSAGE could be used to decide where the robbery should happen, in addition to when.
Role Passing: Deciding Who and Why
Given a simulated world populated by non-player characters, PaSSAGE allows authors to delay the decision of which characters should play the roles in its events. Roles in PaSSAGE are similar to scripts received by the actors of a play; they describe the behaviour and dialogue that a character should perform when cast into that role. By specifying a set of conditions which describe the set of character properties that a given role requires, authors can delay deciding which character should play that role until just before its actions must be performed. As a result of doing so, PaSSAGE gains the opportunity to choose a character for the role based on the player’s prior interactions with various characters in the world. For example, a role created to antagonize the player could be filled by a character with whom the player had previously had an unfriendly encounter.
Story Structure: The Monomyth
To provide narrative structure to its stories, PaSSAGE currently draws from the form of Joseph Campbell’s monomyth, which describes a general sequence of events that makes up a heroic journey . For each phase of the monomyth, PaSSAGE requires a set of events that could potentially occur during that phase; for example, the "Call to Adventure" phase contains events which feature different motivations for a hero (PaSSAGE’s player) to venture out into the world and begin an adventure.
Telling a Story: The Event Cycle
PaSSAGE’s stories begin with an event from the first phase of the monomyth, "Home", during which the player begins to learn about the story’s setting, and PaSSAGE begins to learn about the player’s preferences. When the player completes a phase of the monomyth, PaSSAGE chooses the event from the next phase whose annotations best match its model of the player’s preferences, and begins checking that event’s triggers and searching for actors to play its roles. The event begins when all of the triggers’ conditions are satisfied, at which time the event’s actors assume the needed dialogue and behaviours via role passing. PaSSAGE hints at its estimated "best" course of action for the player as the event unfolds, and updates its model as the player performs actions in response to the event. When the current event is complete, the process repeats, until an event from the final phase is selected and the story is complete.
 David Thue, Vadim Bulitko, Marcia Spetch, and Eric Wasylishen. Interactive Storytelling: A Player Modelling Approach. The Third Conference on Artificial Intelligence and Interactive Digital Entertainment (AIIDE). pp. 43-48. Stanford, California, USA. June 6, 2007.
 David Thue, Vadim Bulitko, Marcia Spetch, Eric Wasylishen. Learning Player Preferences to Inform Delayed Authoring. Papers from the AAAI Fall Symposium on Intelligent Narrative Technologies. FS-07-05: pp. 158-161. AAAI Press. Arlington, Virginia, USA. November 9, 2007.
 David Thue, Vadim Bulitko, Marcia Spetch. Making Stories Player-Specific: Delayed Authoring in Interactive Storytelling. The First Joint International Conference on Interactive Digital Storytelling (ICIDS): pp. 230-241. Erfurt, Germany. November 26, 2008.
 Joseph Campbell. 1949. The Hero with a Thousand Faces. Princeton University Press.