Eye!

What are the sort of things, essential qualities maybe, that you look for in a person when you take his portrait?
You know, I don't. People ask me funny questions like that. I don't direct what I want of them, I let them do it and when I see it, I take it. And if I don't see it, then it doesn't get taken. The thing that's fascinating about portraiture is that nobody is alike.
Ansel once said to somebody that I was versatile, but what he really meant was that I jump around. I'm never satisfied staying in one spot very long. I couldn't stay with the mountains and I couldn't stay with the trees and I couldn't stay with the rivers. But I can always stay with people, because they really are different.
Imogen Cunningham

Dialogue with Photography, Paul Hill & Thomas Cooper

What a great woman! You can see some of her works here.