Simple. Never.
I could natter on about roles and tradition and culture, but the clearest explanation is a demo.
First, put down your guitar. Then, get a fiddler to stand behind you, reach around, stick his fiddle under your chin, and play it.
Loud, huh?
When you're playing in the same range as the fiddler, he can't hear you. What he can hear is your bottom. Your job to make an interesting bass line. He can't do that. It's also to hold down the rhythm and create the chord structure.
The melody? You cannot compete. Everything else? You hold the field.
"Oklahoma Rooster" without BAGDAD? A Bag-dud.
I've heart "that chord," kicking off the B part of "Captain George Has Your Money Come," be an A minor, an F, or -- my favorite, courtesy of Mark Brissenden -- an A major.
Ditto for piano. I've heard beginning contra-dance piano players try to play the melody. Uh-uh. Or tinkle around in the upper register in other ways that are, in theory, interesting. Listening musicians say, "She's playing air piano," and it's not meant as a compliment.
Someone needs to play what we used to call, when I was a kid back in the 1600's, the continuo. The bottom.
I think it's the most interesting part, because you have the most flexibility. The melody is pretty much what it is. You can ornament it or simplify it, but it's never more than a horizontal stream of note, note, note, note, note, note, note.
The vertical part is all you. You control the tune's feel. Throw in a snot-load of relative minors, or a surprising bass line, or swap in swing chords for a time through, and listen to what it does. Better still, do it on stage and watch what happens to the dance.
Truth to tell, I'd rather play backup guitar than mandolin. Sadly, I keep winding up in bands and jams with backup guitar players who're better than I am -- Ron Sommers, Steve Burnside, Linda Askew, Robert Rosenberg -- so I don't much get to. Retreating to mandolin is my only option.
Everyone says the One Great Book for boom-chuck piano is Peter Barnes's Interview with a Vamper. I don't know any comparable book for backup guitar. Or, for that matter, any book at all. I'll try to toss some of what I've learned in here.
Of course, when what you do works the dancers will run up afterwards and tell the fiddler, "That was amazing!"
Don't worry. The other musicians will know. You'll be in demand for gigs, and it all pays the same.
