IMHO – The best thing is to get the song going with an intro, sing a verse (at least one) and chorus, then start to alternate instrumental verses with other sung verses. You can play during the singing, depending on the song but don’t play what the singer is singing. Experiment with a harmony line, or play long notes during the singing and then echo the last few words when the stop. Beware, the biggest mistake fiddlers often make is playing to loud while someone is singing. Save the real volume for your instrumental turn. I hope this helps.