The 1st Assistant Director is responsible for managing the set during production of a motion picture. They are the primary interface between Director and Crew. They ensure that the technical and logistical facets of production are handled smoothly so that the Director can focus on the artistic aspects.
The 1st AD is responsible for scheduling the shoot each day as well as managing the set during production. Good communication skills are key. Shot lists are crucial to the 1st AD performing his or her job efficiently. Without them, the shooting schedule may be created on the fly, and this disorganization will most likely show up on screen.
The Assistant Director's job of calling out "lock it up, quiet, roll sound, roll camera" in advance of each shot.
"Lock it up": On-Set Term; A get ready signal shouted out by the AD, when shooting is about to begin. Because there must be quiet on the set, the phrase suggests that any open doors, cabinets or moving parts on any equipment must be closed, taped, locked or otherwise made immobile in order to ensure silence.
"Picture's up!": On-Set Term; Warning shouted out by the AD that the sequence of cues to shoot a scene is about to begin. Also, a final warning to everyone on set to shut-up.
"Roll Sound": On-Set Term; A call made by the director or 1st AD to cue the sound guy in anticipation of shooting a take. The command always precedes "Action!" so as to allow sufficient lead-time for the audiotape to begin in front of the rolling film.