For me an important part is song structure, esp. if the band plays "interpretations" of originals (Sounds like great fun, actually
). There's nothing worse than missing a stop or starting the bridge after the wrong verse or not expecting that doubled chorus on the last verse. Those are the only places most audiences have a chance to notice. So I'd ask the band if for any of the songs there are significant alterations of the structure compared to the original record (even ask which version do they go off - there is a lot of room for misunderstanding), and clarify the keys they play. Then on the night be in full and constant communication with the rest of the band (I'm sure that you'd do this anyway) and have fun! 
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