The role of a director at an audition is to examine and evaulate the actors and to compose a cast for the show.
The SM usually does everything else. As mc says, your duties could range from receptionist to chief administrator. Some essentials:
-create audition forms for actors to fill out. Sort of like job applications. (include contact info, past experience, availability, etc. Actors usually bring resumes and headshots - attach these to your audition form)
-Welcoming actors and coralling them in a waiting area while individuals audition for the director.
-Creating a sign-up list of audition order
-Setting up the audition space, including a table and chairs for the audition panel (a jar of pencils and some water bottles would be nice)
-Administering the auditions: shuffling actors in and out as effeciently as possible
-Maybe you are asked to phone actors with callback info
-Maybe you are asked to phone actors who don't make the show (this is a depressing job!)
-Making name tags, if the director wants these (I've rarely seen them used, except in group auditions)
-Running the CD player if the audition includes performing to recorded music.
-Snapping digital photos if the director wants "on-the-spot" pics of auditioners
-Cleaning up after
-Keeping your eye on the clock to make sure no auditioner gobbles up too much time. Some people like to hang out after their reading trying to suck-up to the director. You have every right to poke into the room and ask if the director wants to see the next actor. It's only fair - everyone deserves to be auditioned in a timely manner, and you don't deserve to lose your entire afternoon because each actor took a private chat session with the director.
It's an implicit directive that you should try to make the audition environment as calm, creative, and professional as possible. A crazed, messy audition room will probably not help actors do their best, and will take the director's focus away from the talent.
While you're busy making all this happen, you probably won't see many of the actual auditions. So it might be hard for you to comment on the talent (or lack thereof). Most directors I've worked with never ask the SM about casting choices. But a few do. Specifically, they might ask you about how someone looked, about their singing voice, about their personality. I recommend never to offer an opinion unless asked. And the only exception I would make to this would be if someone demonstrated an extremely disagreeable personality under your witness (doing drugs in the audition hallway, for example - only something that extreme). You should share info like this with the director, because such behavior could be extremely destructive to the overall production process.
A few other things -
-If there's singing in the audition, folks will probably ask for a place to warm-up. Have some ideas of where to send them.
-Know where the nearest drinking fountain and restrooms are.
-Be able to tell people if casting will be completed today, or a time by when they can expect to know the results of the audition.
-Be able to tell people the exact beginning and ending dates of the production engagement, and general rehearsal times (evenings til 9? or til 11?).
-If there's dancing in the audition, be sure the waiting area has lots of space for dancers to spread out and stretch.
-If the director will ask them to read from a script, have extra copies (even just of the audition excerpts) available for actors to peruse while waiting.
-It is NOT your job to be a cheerleader, therapist, or gofer for either the director or auditioners. Smoothing ruffled feathers or massaging egos will take too much time away from running the busy auditions. Auditions are emotional, but it's not your job to make sure everybody's happy-happy. It's your job to make the audition happen.