Well actually, as you know, a huge number of folk and country songs are in fact 3 chords--there's even a great book about fairly recent country called "3 Chords and the Truth"--lots of information on Harlan Howard and others.   So I can guess where a song is going and come up with a harmony even if I've never heard the song before---and in that case I definitely wait to hear a verse before trying anything.   And if I guess wrong I change immediately--hoping that nobody caught it. I can always claim it was a passing tone. All I want to do with the viola is complement the melody--and not obscure the words of the singer.   So I can bring the instrument line out at the end of a phrase, but in general I try to stay very much in the background--unless asked to take a break. I find one of the hardest things to play a harmony for several verses then, on a break, to try to play the melody of a song I've never heard before. At least on lots of country and folk songs, the last note is often the tonic. That helps a bit.

The song I cited is called "Is It Cold In Here?", recorded by Joe Diffie about 10 years ago or so now. I'm afraid I don't know how to link to songs.

By the way, Joe, your harmonica is sounding really good.   I would think you could do really well in making up something to play on virtually any train song, for instance. And the harmonica adds a lot to any train song. I wish I could play the harmonica. Never been able to even get a dependable scale.

The 3-chord songs I'm learning are for my wretched guitar skills--since I'd plan to sing them only in places where there will be real guitarists who can bail me out, but who may need to know the chord progression of a given song. I'm gravitating to 1950's and 1960's country these days--"Almost Persuaded", "Begging to You", "Fraulein", "Crazy Arms" .And Lots of Charlie Pride, including "The Snakes Crawl at NIght, "All I Have To Offer You Is Me", "I'm So Afraid of Losing You Again", etc. ... .... All just great fun to sing.