Elicitation and Application of Narrative Constraints Through Mixed-Initiative Planning James M. Thomas Liquid Narrative Group Department of Computer Science Box 7535, NC State University Raleigh, NC 27695 USA jmthoma5@ncsu.edu R. Michael Young Liquid Narrative Group Department of Computer Science Box 7535, NC State University Raleigh, NC 27695 USA young@csc.ncsu.edu Abstract This paper describes a foundation for an interface to allow non- technical human authors to collaborate with an automated planning system to design interactive narrative. Drawing from research in advisable and mixed-initiative planning, a domain metatheory is presented that can encode the narrative goals and preferences of the human author of planned interactive narrative. The authors describe a graphical user interface that exploits this metatheory to elicit authorial preferences. Specific constructs related to interactive narrative are considered to demonstrate how the preferences of the human author may be used to define and control the possible user experiences of an interactive narrative. Introduction “Interactive narrative” describes the stories that develop within virtual worlds in which human users interact with one or more computer controlled agents. The most well known examples of interactive narrative are computer games, but also included are intelligent tutoring systems, embodied conversational agents, virtual environments, and training simulators. A persistent challenge for such systems is the narrative paradox: “how to reconcile the needs of the user who is now potentially a participant rather than a spectator with the idea of narrative coherence.” (Aylett 2000). Few systems attempt to reconcile these goals dynamically at run- time. Those favoring strong plot coherence often restrict the depth of the computer-controlled characters, and/or the human user’s available palette of interactions with these characters, reducing character believability. Systems with interesting and believable characters often lack any automated mechanism to coerce these ‘emergent’ bots to meaningfully contribute to a story. Although many useful and commercially successful systems have been built with these limitations, none has yet met the “Holodeck” standard (Cavazza et. al., 2000). One approach for the balancing of these competing goals is the Mimesis system (Riedl, Saretto, and Young 2003). Their algorithm generates plans for actions of story world characters based on hierarchical task decompositions and discrete causal requirements. Although Mimesis simultaneously solves for plot coherence and character believability, the authors acknowledge (Riedl and Young 2004) that a primary limitation is the lack of a search space heuristic that would allow the system to judge the relative “goodness” of one plan over another. In other words, there is no mechanism to ensure that particular narrative qualities such as “suspense”, “surprise” or “romance” will be produced in resulting plans. One might attempt to define a generalized heuristic function in terms of universally accepted narrative ideals, but most planners lack a sufficiently powerful model to make associations between such generalized ideals and the semantics of a specific problem domain and plan space. Also, no set of heuristics has yet been identified that guarantees “good” narrative even when applied by skilled and motivated humans. As author Somerset Maugham quipped, “There are three rules for writing the novel. Unfortunately, no one knows what they are”. An alternative approach is to involve the human author in defining heuristic functions for each interactive narrative based on that author’s preferences of setting and plot. For the system to capture these preferences and report them to the planner, it must have an integrated understanding of the definitions of actions and entities in the problem domain (the setting) and the effects that the constraints on those actions have in defining the topology of the plan space (plot experiences). A reasonable approach for gaining that understanding is to keep the author “in the loop” throughout the plan construction process. This paper describes the foundations for the design of such a collaborative authoring environment for interactive narrative. The first stage of this environment is being implemented as part of the Zócalo system of planning services at North Carolina State University (NCSU). Planning For Interactive Narrative Planning for interactive narrative offers special challenges and opportunities. The task for planning systems in interactive narrative reaches well beyond finding a single complete and consistent plan. Authors are interested in understanding how unplanned user actions may affect story goals. This in turn raises issues about the variability of narrative experiences that are