Well, there seem to be different backups for different styles. Seems to me the best start would be to do a lot of careful listening, and watch other players, so you can start hearing your part in your head. Don't limit your listening to what the fiddler is doing-- listen to all the parts, because it may be that what the guitar player is doing as a turnaround or fill might suggest a direction. Or the part you might play might be played, on a recording, by any number of other instruments, especially if the lineup did not include a fiddle.

It may also be that the person playing lead can suggest what they might like you to provide. I know that in our gospel band, which I sort of quarterback, developing a common language was required first before Hardi could add what I was hearing in my head.

A good model for hearing and finding good side parts in trad tunes is Barry Taylor's MIDI tunebook, online. His arrangements of trad tunes are wonderful, and the parts vary from full harmony accompaniment to very spare, restrained ornaments. You can open and print these with any notation program (MidiNotate & Noteworthy Composer are simple), listen to the parts together or separately, and find a line that suits you or improvise around the melody, and so forth. Speed is adjustable so that you can noodle around to what works, before the tune has flashed by and ended. You can also experiment with DT and Mudcat MIDI tunes.

~Susan