Mixing Story and Simulation in Interactive Narrative

Simulation is a common feature in computer entertainment. However, in computer games simulation and story are often kept distinct by interleaving interactive play and cut scenes. We describe a technique for an interactive narrative system that more closely integrates simulation and storyline. The technique uses a combination of semi-autonomous character agents and high-level story direction. The storyline is decomposed into directives to character agents to achieve particular world states. Otherwise, character agents are allowed to behave autonomously. When the player’s actions create inconsistency between the simulation state and storyline, the storyline is dynamically adapted and repaired to resolve any inconsistencies. Storytelling vs. Simulation Simulation has been, and continues to be, an important part of computer entertainment. Computer games such as The Sims and SimCity are simulations in the strictest sense. The initial parameters, a model of state change, and legal player moves are the forces that drive the user’s experience. Real-time strategy games are simulations of combat and have been used effectively for entertainment and for military training. Firstand third-person action/adventure games also rely on simulation to a lesser extent. Each mission can be considered a simulation of physics, weapon effects, opponent movements, etc. One difference between action/adventure games and other games such as The Sims or SimCity is the use of story to constrain the player’s experience to a particular narrative path. The most common role of story in computer games is to provide “glue” between missions. Modern computer games consist of interleaved periods of interactive play and cut scenes – short non-interactive scenes that transition from one mission to the next, providing the player with goals and motivation for the next segment of game play. In this mode of alternating between game play and cut scenes, story elements and simulation are kept strictly separate. This is due to the trade-off between control and coherence (Riedl, Saretto, and Young 2003). On one hand, the player wants control to make decisions for the player’s Copyright © 2006, American Association for Artificial Intelligence (www.aaai.org). All rights reserved. character. On the other hand, game designers want the player to experience a coherent narrative progression. Interactive Narrative is an approach to interactive entertainment that enables the player to make decisions that directly affect the direction and/or outcome of the narrative experience being delivered by the computer system. We are building an Interactive Narrative System for training and entertainment that mixes story and simulation. But why use simulation? After all, it has been demonstrated that Interactive Narratives can be constructed out of branching story sequences such as those used by the Choose-Your-Own-Adventure style books. The reason is that simulation provides a realistic, continuous, life-like modality for interaction that can result in a more immersive and compelling experience for players than simple branching stories. The realism of simulation is also beneficial to educational games that provide a realistic learning experience, in addition to a coherent narrative progression.