I’ve filled in for bands variously on guitar, bass or vocals – sometimes two of those things at once.
In my experience, it kind of goes like this:
Most drummers can pretty well fake their way through anything, as long as they can keep a beat. They don’t have to worry about key or chord changes, so if they’re at least keeping decent time, most of the audience doesn’t know the difference.
Bass is the next easiest, in that it’s possible to pare down the basslines to a bare minimum for songs you don’t know. Essentially, you can just thump roots and follow the guitarists.
Guitar, you kind of have to know something. When I’ve filled on guitar, I’ve usually been able to at least get a song list and (for originals) a tape up front, and get in at least one rehearsal or two, even if it’s just during the sound check.
Vocals is pretty easy if you can sneak a lyric sheet onto the stage somewhere. I’ve taped them to mike stands, or down on the floor.
I’ve only done one fill in gig absolutely cold, and without even an idea of a set list. The band did only hair band covers, like Guns 'N Roses, and Poison and Motley Crue, and I did have some degree of a repertoire for those songs, even though it wasn’t what I was really into. I got called to do this gig with them at some bar out in the sticks. I asked them for a song list, and the singer said, “oh, we’ll just wing it.” So I get to the gig, and we did a couple of songs I knew at the soundcheck, and then during the gig, we would just huddle up between every song, and they’d be like, “do you know 'Sweet Child ‘O Mine?’ OK, let’s do that.” Then after that, “Do you know ‘You Shook me All Night Long?’ Ok let’s do that.”
Needless to say, it was a pretty sloppy gig, but these guys were lackadasical slackers just in it to try to get laid.