I get a mental picture of the characters, but it is rarely anyone famous. I try not to get overly descriptive because it's usually not necessary. A characters appearance changes according to the reader anyway. In the Man In The Woods, here on lit, the characters were based on me and my ex-fiancee. Except it was when I was younger and had long hair, instead of the shaved head.
The one exception to my nobody famous idea is in my novel. I couldn't help but picture Jack Black as the main character, Lenny Kapowski. And now, since the idea of making my book into a movie has come up, my friends and I have been having fun casting the characters. I did kind of lazily describe some of the characters in the book by saying they looked like famous people. "Tony Bennett's evil twin" and " a meaner version of Ed Asner" come to mind.
One thing I am getting a kick out of when I discuss my book with people who have read it is the differences in how they see certain characters as compared to how I see them. It's really come out in the casting discussions when someone suggests an actor that is light years away from what I thought. But that's part of the coolness about books.