Some people are much harder to portray than others. All those whose livelihood depends on projecting a certain image are almost impossible to portray well. They simply aren’t willing to give anything other than that which they want to give, and no good art is possible here. I’m tempted to think that we are all moving a bit into that direction, given we are so used to switching on our photo faces in the presence of a camera (just look at photograph in Facebook albums, and you know what I mean).
It would thus seem that for a good portrait you need a photographer who knows how to take, but you also need a subject who knows how to give. Maybe the art of the skilled portrait photographer lies just there: To find those very people who will be able to give just the right amount. Asking just the right people for the photographic dance or - if you prefer your metaphors a bit juicier - for the photographic coupling.