Mobile experiences that take place in public settings such as on city streets create new opportunities for interweaving the fictional world of a performance or game with the everyday physical world. A study of a touring performance reveals how designers generated excitement and dramatic tension by implicating bystanders and encouraging the (apparent) crossing of normal boundaries of behaviour. The study also shows how designers dealt with associated risks through a process of careful orchestration. Consequently, we extend an existing framework for designing spectator interfaces with the concept of performance frames, enabling us to distinguish audience from bystanders. We conclude that using ambiguity to blur the frame can be a powerful design tactic, empowering players to willingly suspend disbelief, so long as a safety-net of orchestration ensures that they do not stray into genuine difficulty.