Ask them what they didn't like about 30 other bassists they tried out. Also, find out what kind of personality they want in a bandmate.
When they ask your opinion on matters, music-related and otherwise, don;t be afraid to express them, but make sure they are your true opinions. The last thing you want is to give a false impression and then having to live it up to it later.
Remember that you are going to have to "live" with these dudes during practices and performances and you want to make sure that you are a good fit (and vice-versa), musician-wise as well as personality-wise.
On a less-than-serious note, be sure to walk into the audition with a boombox held high above your head that's playing Peter Gabriel (a reference to the band's name).
