Assistant Directors are responsible for the assembly of all the elements needed for filming and for the daily operation of the shooting set. Their objective is to provide the Director with everything he or she needs to put his or her vision on film. Their duties are supervisory, organizational, administrative - and multifarious.
Working within the structure that is governed by budgets, union and guild contracts, industry custom, and so on, they make schedules, attend to the cast, direct extras, oversee the crew as each shot is prepared, create detailed reports of each day's events, among many other things, and are looked to by cast and crew to solve the many problems that continually arise.
The First Assistant Director is the Director's right hand. The Second Assistant Director is the First's right hand. The Trainee is a member of that team and learns by active participation in the process.
Assistant Directors are not Directors in the making. Although some Assistant Directors have become Directors, the managerial/administrative road more often leads to becoming a Production Manager and/or Producer. In fact, being an Assistant Director is a career objective in itself; it is a highly revered, lucrative position with excellent benefits that enjoys considerable status in the industry.