Action Replay (aka Instant Replay): Two performers enact a given scene, usually making exaggerated physical movements; the other two observe visually, but wear headphones so they can’t hear the scene. They must then attempt to create their own scene, re-enacting the same motions and concocting a scene to fit them. Colin Mochrie and Ryan Stiles typically are the reenacters.

from http://en.wikipedia.org/wiki/List_of_games_from_Whose_Line_Is_It_Anyway%3F

Yay for Whose Line!

It is called an act.