Have a look at
Heidi Hollinger. This girl is amazing. She has a studio on Sherbrooke W. You should see the equipment: basically nothing. One of those pesky umbrellas, and half of the time not even that.
Into Galleries/
Then Celebrities, Politicians and Corporate.
You'll see how casual everyone is. And that's exactly the point: aside from Celebrities (most know how to pose), some politicians and most business guys are clueless about it. Yet, they're all natural.
Bottom line: I think it's more the photojournalism kind of photo that makes the best ever portraiture. Camera in hand and screw the heavy studio setups.