It refers to a lower level director who works simultaneously with a small subordinate crew in a second unit under the supervision of the director and the first assistant director. The second unit director is responsible for filming shots of less importance, such as inserts, scenes where the major actors are not present, crowds, scenery, etc. and may be assigned particular responsibilities for supervising and carrying out such activities as casting, dialogue, keeping track of day-to-day changes in scripting, etc.