I don't spend a lot of time thinking of what they'll do musically, I try to imagine being locked into a windowless room with this person for twelve hours at a time. If you can look at that and think it might be fun then maybe you've got the right musician. ↗
It's true that the more you put in the more you get out and that has to be there I think, If you aren't really hooked on your instrument this job would be a hell on earth but if you are, it's the best. ↗