Stand-ins in film are often misunderstood to be doubles for the actors, that is, people who double for the actor during filming, e.g. from behind, in makeup, or during some dangerous stunts. Stand-ins are not the same as doubles, and do not appear on camera. Stand-ins are people who stand in for the actor before filming, while the lighting is set up. Lighting setups on a film can be a slow and tedious process; during this time the actor will often be on the sidelines watching the setup, or somewhere else studying his or her lines.

Stand-ins must not necessarily look like the actor, but they must be the same height and build as the actor so that the lighting in a scene will be set up correctly (for instance, if the lighting is set up with a stand-in shorter than an actor, the actor will have his head in relative darkness).