I think it all depends on the type of gig, amount of time you have spent with the group, and what resources are available. I have recently joined a covers band. We have a rehearsal tonight, and a gig on Saturday. My goal is to 1. get the chord progressions and style right..then 2. do the appropriate riffs in the right places...then 3. start to incorporate unique things the band does with certain songs, and also to be able to anticipate and react to mood changes in each song. I think this is a process, and takes a while to really get comfortable with in a new group.
For jazz or legit music, I can sightread and always have a lead sheet or the printed part in front of me. If it is really difficult, or just written weird (a lot of contemporary big band stuff), then I learn that in chunks so I am not really reading every note, just the phrases.
FWIW, I really don't like improv jam sessions with Real books...I find them boring and just full of ego and fluff. Just not my bag..I do it to be polite.