A system to manage human interaction in immersive environments was designed and implemented. The interaction is defined by an "interval script" which describes the relationships between the time intervals which command actuators or gather information from sensors. With this formalism, reactive, linear, and tree-like interaction can be equally described, as well as less regular story and interaction patterns. Control of actuators and sensors is accomplished using PNF-restriction, a calculus which propagates the sensed information through the interval script, determining which intervals are or should be happening at each moment. The prototype was used in an immersive, story-based interactive environment called SingSong, where a user or a performer tries to conduct four computer character singers, in spite of the hostility of one of them.