I've been snapping some in-progress shots of this piece, and I'm posting a few here.
The method I'm using for this is called grisailles (that's French for "gray", pronounced "grizz-AYE"), so called because the values are worked out first in monochrome shades, and then color is glazed on in stages after the underpainting is complete and dry. It is a very old, traditional way to work. I first discovered it and tried it out in college, after studying some of Leonardo da Vinci's work. This was the method he chiefly preferred.
This method works especially well, I think, for portraits from black-and-white photos, which this happens to be. The background street scene is from another photo, and depicts London in the 1920s, which is around the time Chesterton was writing and was popular in England. A household name, in fact.
I don't know if a purist would - properly speaking - consider this true grisailles, because the toned ground is a warm brown, but the principle is the same; work out the values, and then glaze in color.
I do a lot of subtle adjusting as I go. A lot. In the beginning color stage, for instance, I fattened GKC's nose a bit, as it was to thin in the underpainting. I've also muted the background, which I'd planned to do from the outset. I don't really want it drawing the eye very much. It's all about layering from here on out, though, so just about the whole image will continue to develop.