Interest has been growing in the behavioral animation of autonomous actors in virtual worlds. However, authoring complicated interactions between multiple actors in a way that balances control flexibility and automation remains a considerable challenge. A proposed behavior-authoring framework gives users complete control over the domain of the system: the state space, action space, and cost of executing actions. To specialize actors, the framework uses effect and cost modifiers, which modify existing action definitions, and constraints, which prune action choices in a state-dependent manner. The framework groups actors with common or conflicting goals to form a composite domain, and a multiagent planner generates complicated interactions between multiple actors. The Web extra is a video that shows how multiactor simulations should aim to strike a happy medium between the automation of generation and the flexibility of specification.