Doing the watercolour forces me to look at what is actually happening on the paper. The marks turn out so differently from the expected and actually get a real sense of solidity quickly (paradoxically to the thinness of the water paint), so things actually happen quickly and change often, making me thing of the idea behind the painting more and how it changes with the changing marks on the paper.
With the oil, doing it afterwards, I lost some of the tendency to build up the paint thickly to get a sense of solid flesh - there's a lot more rubbing the paint into the canvas to make a sort of opaque glaze for the mid shadows, and I used a lot more linseed oil in the dark shadow areas. Only the really highlighted areas were scumbled on more thickly. I also used more lines to help define the edges and contours, so as to leave the paint in a more suggestive form - which is really satisfying to me - and not to have to overwork it to make it appear real.
I have been looking at Rembrandt (for ages I haven't like his work at all, but suddenly came round to it again), and it seems like he did an illusion of an illusion - i.e. using quite thin, short-worked paint in such a way as to give an illusion of thick paint recreating solid flesh. Some of his paintings seem to have been such arduous jobs, but I think they are in fact quite simple, with lots of of small suggestive lines, and intelligent compositions as to just seem arduous, but actually quickly achieved. With the work enjoyably in the idea behind the painting and not with the pursuit of technical success.