A couple of years ago I visited the Art Institute of Boston at Lesley University as the guest of the illustration department. After giving a lecture, I did a portrait demo of painting professor Tom Barrett.
As you can see, there were at least three sources of light (two incandescent floods on stands and the window light, but the one next to the table predominated. The drawing is in water-soluble colored pencil and watercolor about 8 x 6 inches.
Once I had established the preliminary drawing, one of the earliest decisions was where to place the highlights, since I was using transparent watercolor only. So when I laid the first washes of skin tone across the face, I painted around the white spaces on the forehead, the tip and bridge of the nose, and the cheeks.
In watercolor, since highlights are the lightest values, they must be left as unpainted white paper. They can also be masked out from the start, or applied with gouache at the last. In oil paintings, highlights should generally be saved for the last.
There's a lot more on the subject of "highlights and specularity" in the six-page article that I wrote for the current issue of International Artist magazine, which should be on the newsstands now.
Flickr stream with more photos of my visit to AIB
Art Institute of Boston at Lesley University
International Artist magazine, Issue 90
Photo above by Keith MacLelland
Media: watercolor pencils and water-soluble pastels. Water and ink applied with water brushes, one filled with water, and the other filled with fountain pen ink.This subject is also covered in my book Color and Light: A Guide for the Realist Painter (Amazon), also available signed from my website store.