It depends on what they ask you to do. Years ago, I used to work at Knott's Halloween Haunt in California. My first audition, I was asked to act like a shapeshifter. I had like 1 second to think about it. I ended up screaming some help me's as I contorted to the ground and "shifted" into a snake creature where I slithered around on the ground and hissed and screamed.
Kind of weird I know, but I got a part.
Ok, so they'll tell me what to do? When I was on the website for it, it said to come in costume if you wanted to come in costume you could, so I thought that you would have to come up with something to do.
CF - I'll throw this out - if they suggest coming in costume, they msay want to see how easy it is for you to "get into" the part. You just might want to get into character before the actual audition. Do you know what type of characters they've used in the past? And do they want more of the same, or something different?