If you are ever lucky enough to play with a fiddler who understands Bluegrass, all you have to say is, "Just play Chucks and fills"
The fiddler will then give you a rhythm accompaniment on low, often double stopped strings (on the off beat, using short staccato bow strokes) and melodic ornamentations and musical comments at the ends of the lines and the gaps between the verses.
What more could you want? A good player will get it perfect on the first take, and there is no competition between fiddle and vocals.
Cheers
Dave