Many computer games use custom scripts to control the ambient behaviors of nonplayer characters. As a result, story authors must write computer code for the game world's hundreds or thousands of NPCs. Creating entertaining, nonrepetitive NPC behaviors without investing substantial programming effort to write customized, nontrivial scripts is challenging. Current computer games have simplistic ambient behaviors for NPCs; it's rare for such characters to interact with each other. To address this, we developed a model that uses generative behavior patterns, which let authors quickly and reliably generate realistic and entertaining NPC scripts. The approach has proven successful, even in the challenging area of creating interactions between NPCs. This article is part of a special issue on interactive entertainment.