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 is not a mystical thing, however, it is obvious and practical and I think that what the performer does is to try to get to that point with every choice you make from the phrasing in a tune to the choice of tunes.