Here's a pencil portrait of the Irish fiddler Larry Reynolds, drawn while he was performing with Seamus Connelly in 2003.
What I was trying to accomplish was a principle that I call "enveloping tone." It's related to the concept of "sfumato," the smoky atmospherics made famous by Da Vinci in works like the Mona Lisa.
Whatever you call it, the idea is to make the a strong tonal contrast between the illuminated area and the shadowy areas. At the same time, the transition from light into shadow should be fairly gradual.
The most important thing to keep in mind is to try to keep the light areas unified, without being interrupted by too many dark accents. And the dark regions should be mysterious, like an ink cloud from an octopus, swallowing up light accents into its enveloping shadow.
The early Daguerrotype photographs had this quality of enveloping tone. They're like a bathtub into which someone has poured condensed milk in one area and India ink in another, with the two principalities blending and merging into each other.