My husband The Gorilla is in the homestretch of shooting his third feature film, a 3D installment in a franchise he created. This is my End Credit series, where I explain what all those titles mean at the beginning and end of a movie.
As it sounds, the AD works directly with the Director - who we have established is the captain of the ship - but unlike it sounds, the AD doesn't direct in at all the same way that the Director does. An AD's direction is about logistics, not art. The AD's work on set largely involves playing the "bad cop" to the Director's Chief of Police. (Wait, are we on a ship or at a police department?) The AD's "bad cop" duties include regularly barking orders to insure that everyone is running on time and in the right place before someone calls "Action."
The AD coordinates all of the elements to get the shot ready for the Director. Camera, sound, props, extras, you name it, the AD has them doing their thing when they're supposed to be doing it. He also has them cleared when they need to be cleared. Since he is in charge of all these various elements, his awareness of the Director's pace as well as the producer's timely budget concerns is of the utmost importance.
The AD and those from his department can usually be heard hollering for or at the other departments, checking that they're ready and checking that they're where they're supposed to be. They are the keepers of all things schedule and their loud voices are also lent to call "Action" and other such things for the Director. On set, if a crew member has a direction-type question, they will approach the AD department and not the Director.
If the production is large enough, there will be a full AD department led by the First AD over the 2nd AD, 3rd AD, and so on. AD's need to be very organized, not afraid to get mean, and can't be meek-voiced.
A quality AD really has the pulse of the overall production. It's a more prestigious position than it sounds, and people make entire careers as Assistant Directors.
*Itching for more End Credits? Click the box in my left sidebar that says "The Gorilla's Movie" for a list of all the title's I've covered.