Setup happens before every scene—and generally follows a five-step program inherited from the Hollywood studio system.
Even if you're working on a smaller-scale production with less formal procedures, you should know how the drill goes on a studio lot.
As preparation begins for a new scene (which may or may not be on a new set), the director claims, or assumes authority over, the set. He sends the technicians on a break so they can make personal calls, grab a snack at the craft services table, or gossip. He dismisses the crew so he can discuss the scene with the actors and give them his full attention, without being distracted by technical questions. Stand-ins observe.
The actors run through the scene once ...