The play goes on without anything catastrophic happening that wasn't staged.
The audience immediately realizes when something wasn't supposed to happen and react realistically.
Enforced: "How can the heroes save a building full of people from zombies without breaking the Masquerade? I know—the zombies attack during a play, and everyone assumes that they're actors!"
Lampshaded: "Are those people blind? How can they not realize we're being attacked by real zombies?"
Invoked: The director insists to the audience that everything that's happening is all special effects and that everyone should remain calm.
Exploited: The Evil Overlord sends his zombie minions to attack in the middle of the show so that no one will realize what's going on until too late.
Defied: The lead actor turns to the audience and tells them that the zombies are not a part of the show and they should all exit the theater as quickly as possible.
Discussed: "What do you bet that the audience thinks this is just a really unconventional staging choice?"
Deconstructed: Zombies invade the theater, and the audience believes it's just a part of the show. Nobody calls the police or the military and nobody runs away. The zombies infect them all and begins the zombie apocalypse.
Reconstructed: The hero arrives before the zombies can attack and defeats the zombies while posing as an actor in the show, thus defeating the threat without alerting anyone in the theater that they were in any danger.