Richard Avedon (1923-2004) in my opinion is probably the greatest portrait photographer ever. It has been said of him that he didn't take people's picture as much as he captured their soul. After seeing an exhibit of his works at the Phoenix Art Museum http://www.phxart.org/ this week, I believe it.
There were prints of his fashion photography for Vogue, Harper's Bazaar and other publications, and while very nice, they were not near as compelling as his portraits. He violated every rule of portraiture in making his shots, shooting very close in at times, "square" shots, one light source, etc., yet every one of them evoke the essence of the person to life. From the insecure smile of Marilyn Monroe to the insincere smile (read smirk) on the face of George Wallace, he does indeed capture that moment when the subject isn't posing, but rather is caught in that instant that they are simply being themselves.
I don't really do much portraiture, but after seeing this exhibit, I think that's about to change. I feel a draw here, an urge that this is an area I want to experiment more in. He set an incredibly high bar though. If I'm ever good enough to be even half as good as him, I'll have accomplished much.
Namaste,
Jill