[2] The director, designers, and crew may have already run through the technical elements of the show without the actors, also known as a dry tech.
Actors have their lines memorized; lights, sound, scenery, and costumes are fully designed and completely constructed.
This allows the actors to become familiar with the set and costumes, the technical production crew to iron out unforeseen problems, and the director to see how everything comes together as an artistic whole.
A set door that performed fine the week before may bang shut too loudly now that there are live microphones on the stage.
For both the technicians and actors, it is the most hectic part of a show's run, as they are forced to do a massive amount of work getting timings and cues correct, often without having seen the scenes in their entirety.