I don't know about Alton Towers, but I know that auditions at other places and even in things like TV & Film, you want to make yourself stand out from the crowd and be memorable. And that's certainly what I'd be looking for at the auditions for my scare attraction this year.
If it was me going for an audition, here's what I'd do. I'd invent a character. Nothing too costume heavy but something simple and suitable for halloween and maybe just a touch of makeup. Get into that character the moment you arrive and stay in character the whole time you are in front of anybody, except when asked to do something specific during your audition though. But when staying in character at other times don't be annoying and a show off.
There was a girl who used to work in ToTT back in the old days and her character was called Esmeralda. She now works at Tulley's and has become one of the most famous characters in the scare industry. She would never break character though - going for breaks, going around the resort, even when being interviewed. If the costume was on, she WAS that character.
Here's a video from the auditions at Blackpool Dungeons http://youtu.be/_oAkknyutPY
so you have an idea of what to expect. Similar thing going on here.
Whatever you do just absolutely try your best to stand out and be memorable when you're up against lots of other people, but memorable for the right reasons! At somewhere like Alton Towers, the more fun and entertaining you can be, the more they'll love you!