Why it comes first
The master sets the rules. When the director and actors finish blocking a scene, the master is the first place that blocking gets filmed in full: where Maya sits, when she stands, which hand holds the coffee, what line Joe says as he crosses to the window. The script supervisor notes all of it. Every close-up and over-the-shoulder shot afterward has to match those actions, or the editor can't cut between them.
Lighting follows the same logic. The DP lights the widest frame first, since that's where you see the most of the set, then refines for tighter shots on that side of the room.
An example
Scene 31, INT. APARTMENT - NIGHT, 1 6/8 pages. Three roommates argue in a living room.
The director stages the master from the kitchen doorway on a 25mm lens, seeing the couch, the window, and the front door. It runs the whole scene, about two minutes. The company shoots five takes. Take 4 gets circled.
Then coverage: a three-shot on the couch, singles on each roommate, and an insert on the eviction notice. Seven setups total, with the master as setup 31 on the slate.
Common mistakes
Shooting a master nobody will use. A 3-page scene staged in a cramped car interior may never play wide, and spending an hour lighting a master for it costs the setups the scene actually needs. Directors who plan in a shot list can skip it on purpose instead of by accident.
Blocking drift. By take six of the master, an actor finds a better moment and crosses earlier. That's fine if the script supervisor notes which take is the reference for matching. If nobody decides, coverage will match take 2 in some shots and take 6 in others.
Rushing through the master to get to the close-ups. On a long scene, the master is often where actors find the scene, and some editors rely on it more than directors expect.
Masters and scheduling
Masters take longer than their page count suggests because lighting is at its widest and every department has something in frame. When the 1st AD estimates time, the first setup of each scene or each side of a scene carries most of the lighting load. Wide shots also show more of a location, so the art department's set dressing and any background actors have to be ready before the master rolls, well ahead of the tighter coverage.
