Wikipedia definition

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A stand-in in film and television is a person who substitutes for the actor before filming, for technical purposes such as lighting.

Stand-ins are helpful in the initial processes of production. Lighting setup can be a slow and tedious process; during this time the actor will often be somewhere else. Stand-ins allow the director of photography to light the set, the camera department to light and focus scenes. The director will often ask stand-ins to deliver the scene dialogue ("lines") and walk through ("blocking") the scenes to be filmed. In this way, a good stand-in can help speed up the day's production and is a necessary and valuable cast member on a film.

Stand-ins are distinguished from body doubles, who replace actors on camera from behind, in makeup, or during dangerous stunts. Stand-ins do not appear on camera. However, on some productions the jobs of stand-in and double may be done by the same person.

Stand-ins do not necessarily look like the actor, but they must have the same skin tone, hair color, height and build as the actor so that the lighting in a scene will be set up correctly. For example, if the lighting is set up with a stand-in shorter than an actor, the actor might end up having his or her head in relative darkness.