They are recording all your friends and know who's doing what. Don't oversell yourself but be confident and assertive. Find out what they are looking for in a musician and determine whether or not you fit the criteria. The second most important criteria in getting a gig is how well the other members of the band think they might be able to get along with you. Be friendly, but be yourself.
People can usually see through facades.
It's even more important after getting the gig to be cooperative and extremely diplomatic. You will have opinions, but try and not be to forceful with them. Keep a positive attitude and you will have respect from the other players. Wear a smile and act like you're having a good night even if you aren't. Be a strong force in the group.