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.
When the audience is awful you can still have a great night and people will walk out thinking they had a great time even though there was loads of loudmouths and the sound was terrible.
I think if you are writing an instrumental you are dealing with more of an aesthetic in a sense but a lyric is more of a putting yourself on the line and a much more expensive exercise.
I think that open tunings are a trap really because it's really hard not to sound like an open tuning when your using one and that gets old as well as what you learn in one open tuning is going to stay there.
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.
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.