I may not be very qualified to address this question since I basically don't play fiddle--only viola---but I am used to making up harmonies--as I'm sure you know it comes with the territory. Admittedly I can only do it when the song or tune is slow (waltzes are particularly easy to find harmonies in), but there are a few tricks I have found useful.

1) As has been said before, avoid at all costs playing the melody--that adds nothing---unless of course you have the go-ahead for a break.

2) Try to figure out quickly what key the tune is in (or ask before).

3) Don't be afraid of stark open chords.

4)As has been said before, countermelodies are fine, often even welcomed. Try to avoid just 3rds or 6ths, although they have their place.

5) Often the best time to come out of the musical texture is the end of a vocal or melody line, when the singer or melody instrument is pausing. But don't forget to retreat when the next verse starts (or go back to counter-melody)

6) Don't be afraid of roaming up and down your instrument--different octaves (though I lean toward lower harmonies), different rhythms (this is not of course or upbeat bluegrass or upbeat fiddle tunes) while still keeping the underlying rhythm of the song and still ending on a note that fits the chord or the song.

7) Most importantly, trust your ear. If your ear tells you your musical idea is wrong, change immediately. I make wrong guesses often, but then I change, trying to change so fast it sounds like a passing tone.


Have fun!