Chill dude. Really. The more tense you are, the more you're going to look nervous and tense. And the more likely it is that you're going to walk out of that audition thinking "FUCK! That is not at ALL what I was setting out to do."
It is, because we've all done it.
The biggest reason you need to relax is that it's not that big a deal. It's not surgery, or a real gun fight. Honestly, it's just fuckin' around, it's playing. It's just that if you land it, you get paid for it.
Relax, and tear that shit up.