(Article from http://en.wikipedia.org/wiki/Assistant_director ) 

An assistant director (AD) is a person who helps the director in the making of a movie or television show. The duties of an AD include setting the shooting schedule, tracking daily progress against the filming production schedule, arranging logistics, preparing daily call sheets, checking the arrival of cast and crew, maintaining order on the set, rehearsing cast, and directing extras. In Australia, the first assistant director may carry a legal responsibility for health and safety as the "responsible person" and representative to the producer on set.[citation need

Historically the role of an assistant to the director was a stepping stone to directing work; Alfred Hitchcock was an AD, as was James McTeigue. This transition into film directing is no longer common in feature films, but remains an avenue for television work, particularly in Australia and Britain. It is more common now for ADs to transition to production management andproducer roles than to directing. The role of AD has also expanded to become a separate technical profession.