Usually, I like to get a really detailed drawing going on before I sit down to work. That didn't happen this time and I had to move to the painting early.
I remembered something a teacher told me once that process was a way of idiot proofing your work. This is partially why I use a black and white underpainting first. It helps me set my values and make sure I only have to focus on one thing at a time. Thumbnail and drawing focus on composition, painting focuses on value. After that I paint over it with color layers and anything else I think will allow me not to screw up the hard work before it.