Quick question: What shirt did you wear yesterday? If you can’t recall, you might need a script supervisor.
Movies aren’t shot in the order they’re viewed. Instead, scenes are shot in the most efficient order. So days—even weeks—can elapse before a scene that completes an earlier sequence is shot. It is the script supervisor’s job to ensure that all the elements of the two scenes match.
There is the matter of getting all the costumes right, but the script supervisor must also track things like the camera settings, prop placements and even the eye angles of the actors. They must also reset the scene back to its original state when the director wants to do another take.
A script supervisor has several ways of keeping track of things. They take photographs to record the visual elements of a scene and each costume. They also prepare a continuity breakdown and take notes to record vital information. All this will be indispensable when it comes time to edit the film.
The script supervisor is represented by IATSE.