Here I begin by filling selections with flat colors. Looks rather gross to start, doesn't it?

I do the main body on a separate layer, for two reasons.

I can make global changes to the character or background to enhance mood or lighting without having one layer affect the other (though if I paint properly and use color well, I won't have to).

The other reason is I can turn on Preserve Transparency in the Layers Palette. If you're not familiar with this, what Preserve Transparency does is allow you to change the colors of pixels on a layer that has color in it in the first place. If an area is transparent, you can't add color to it, or change it. If an area has color you can paint over only the colored parts. It's like having an automatic selection without using memory for an alpha channel with a saved selection.

Keep in mind that I'm using a pressure sensitive pen & graphics tablet, with photoshop tools set with Opacity on.

I start adding highlights, shadows and textures, using the airbrush to roughly block in large areas. But, for the bulk of the painting, I use mainly the pencil tool (Anyone Surprised??!!)