Internet Filmmakers' FAQ
The script supervisor basically keeps track of things. First of all, for the production office, he or she keeps track of number of pages and scenes covered in a day, the number of setups, the estimated screen time, the official lunch and wrap times, etc. Secondly, for the editor, he or she keeps a detailed list of shots, including type, number of takes, prints, film and sound roll where they might be found, etc. Finally, and more generally, the script supervisor is a kind of clearing house for all the details associated with film continuity.
For example, the wardrobe department is responsible for keeping track of the number of buttons open at the top of an actor's shirt during a scene, but the script supervisor also keeps track of this, and, if there is a question, tries to help out. An actor is, more or less, responsible for matching his or her action from setup to setup, but the script supervisor also keeps track, and can be called upon to help out. The director and the DP are responsible for keeping track of screen direction in the coverage of the scene, but the script supervisor also has a backup responsibility in this area. And so forth. Also, the script supervisor keeps a copy of the script handy at all times, so that if anyone needs to refer to it - an actor for lines, the director to see how this scene is linked to the following one in the script - it is available.