I hardly ever start painting right off the bat. A three hour work session broken into 20 to 25 minute blocks allows time to figure out composition and shadows, to slow down and see. The first time block, for me, is always devoted to pencil thumbnails. Block two is for putting the figure on oil paper (or canvas or panel or whatever), checking proportions, correcting. Then I’ll actually brush on some paint from there onward.
One thing you can’t control for is the model’s inner life, and how it expresses outwardly over the course of the figure session. If an interesting look lights up her face, it soon vanishes, like a bird on a lovely flower. If you dare, ask the model what she was thinking about right at that moment. Maybe, if you’re lucky, you’ll see it again. Or, maybe you’ll see an even more interesting expression. Now, draw quickly. Very, very quickly.