I saw the Leonardo exhibition at the National Gallery in London over the holidays, which was fabulous, though reminiscent of looking at great art on a rush hour tube, so crowded was the space. I was struck by how significant two pieces of improv practice were in the masters work - collaboration and being willing to be changed. All of Leonardo's major works (with the exception of 'The Last Supper') have at various times, been attributed to his pupils. And even though the Madonna Litta, for example, is (currently) attributed to him, it really would seem to be a collaborative work, as the studies by Boltraffio that surround it so beautifully demonstrate.
The other thing was a comment by the gallery's restorer, Larry Keith, on the audio commentary (which I would highly recommend if you go). He mentions that despite doing a lot of preparatory work, Leonardo was very willing to let the final work change as he painted it, even if it meant the original ideas were transformed beyond recognition. He wrote about this and encouraged his pupils not to get stuck on what they had prepared.
My favourite piece was the 'cartoon'. I am not the only one. Apparently people flocked to see it when it was first displayed. It makes me think that 'finishing' things is over-rated.