Inspired by Natural Light
In many cases, the photos are lit specifically to mimic the existing, natural light -- albeit in a very controlled way.
In a blog post, Seattle photog John Keatley walks through his process of working with natural light as his inspiration. First, study the natural light. Then design your key to mimic it. Then fix all of the tonal issues that keep your camera from seeing a scene the way you eye does.
John Keatley uncorks a full, OA-style post, here.
[EDITOR'S NOTE: Strobist published an extended post on John Keatley, including walk-thrus of two portrait sessions, earlier this year.]