Steve, I have the opposite problem -- I can play it once I get rolling but I can't hear it in my head. Makes it hard to get started!

Susan, part of what you're hearing might also be ornamentation. Little notes outside the melody, so to speak. They don't show up on the sheet music much, because they are hard to notate and thend ot e made up as they go along.

I think of them as lead material, mostly, but I could see how they would be effective as "fill" in backing material. You hear that a lot in Celtic fiddling. Some ornaments would be rolls (long and short), cuts, bowed triplets, bends, etc.