I couldn't think of a more laconic description. Two characters meet for the first time onscreen, and begin acting a certain way towards each other. They may act like they don't know each other, or they're enemies. All of a sudden one of them says something that makes you realize it was all staged.
They two of them could be joking around, or they could be doing it for the benefit of a third character.
- Butch Cassidy and the Sundance Kid: Early in the film, Sundance parts from Butch for the night. We then see him sitting alone in a house. A woman enters, and it appears to the audience that he's ordering her to strip at gunpoint. Then he walks up to her and she says "You're late!" and embraces him, making us realize they were just fooling around.
- Runaway Jury: Nick enters an apartment that's not his. A woman he'd met earlier confronts him with a baseball bat. At first it appears he's some kind of home invader, but then he makes a personal comment about his bat and we realize they're actually accomplices.
- A one-sided version happens in The Empire Strikes Back. When Lando first walks out to meet Han and company, he acts like shit's about to go down, then suddenly embraces Han and laughs.