On a spring day in 1990, in a tiny studio theater at Carnegie Mellon — the first university in the world to offer a degree in drama — a most unusual performance took place. The seats were empty except for a handful of computer science researchers, and the only audience member was on the stage. She stood amidst a minimalist set representing a bus station along with a small troupe of improv actors wearing headsets. She’d been told she was taking part in an experiment in “interactive drama,” but he...