Richard Avedon said, "A photographic portrait is a picture of someone who knows he's being photographed, and what he does with this knowledge is as much a part of the photograph as what he's wearing or how he looks. He's implicated in what's happening, and he as a certain real power over the result."
I've often thought that portraiture was one of the most interesting parts of photography. I once made the statement that I only wanted to photograph people who were willing to let me photograph them the way I wanted to photograph them, not the way that they wanted to be photographed. Talk about arrogance; this is not about you but about me.
I also think that great portraits make you want to know more about the person. You shouldn't have to know them to make the picture interesting.
An interesting question has always been why are we doing to portrait? Whose itch needs to be scratched? I frequently see people that I think would be interesting to photograph. They have interesting faces. However, a great portrait takes participation by the subject. I think this may have been what Avedon was suggesting.
I'm not sure that portraits ever lie. What is recorded is open to interpretation by the viewer, but what was in front of the camera is what is recorded. The spin that we place on the image is a product of our own baggage.