Be prepared to meet some strange people. The first audition I went to, there was a guy who sat against the wall and talked, but didn't speak to anyone in the room. He didn't get the job.
Also, remember that the haunt you are going to audition at is (presumably) a professional establishment. You don't have to dress like you're going to a job interview, but don't wear clothing that makes you look like a reject from a slipknot video, either. They'll probably ask you to exhibit something simple, like feigned fear or false anger. Be prepared to have them observe every move you make, as well as facial expressions. I would suggest leaving your cell phone in the car, or turning it off.