The preproduction period is hugely important to a film's creation, and the first assistant director (1st AD)'s ability to plan, plan and plan some more is critical to long-term success. Obviously, certain uncontrollable factors will screw things up from time to time, like an unforeseen rainstorm during an outdoor shoot or the death of an actor during filming, like Philip Seymour Hoffman. However, the 1st AD's ability to organize and rework the schedule as efficiently as possible can make or break deadline dates, saving money, time and sanity all around.
On top of keeping the ongoing schedule, the 1st AD must also create and maintain a database of who worked when (for pay purposes), comb through the script to identify prop or special effects needs, and then make sure they come to fruition.
Once production begins, the 1st AD has the unenviable task of keeping the crew aware of the day's production points and on track, time-wise. He must also make sure union rules and location agreements are followed. Blocking (where the actor stands as he would in the scene and the lighting and sound people figure out where they need to be), rehearsal and on-set communication are also critically important job responsibilities [source: How To Film School]. Although this is a high-visibility role with lots of opportunity for growth and advancement, it's also a high-stress position with opportunity for ulcers and anxiety.