Second Assistant Director. Responsible for getting actors to set as required. Schedules makeup, hair and costumeso that the actor is ready to be filmed at the correct time. The "second" will also liase with and schedule supporting artists or "extras" to make sure that they arrive on set on time with the appropriate hair, costume and makeup for the scene being filmed.
Like the 1st AD, the "second" will constantly adjust the day's schedule to react to changes and any over runs that happen on set and work to keep the day running smoothly.