I'm not an expert, but what I've learned from my own work is that you have to be a little in love with the subject.
It also depends on what you're trying to capture. My most successful pictures never look like the photographs, but, and I hope I'm not flattering myself here, there's a psychology in every one.
In my experience, every picture is a portrait of the artist at the time that it was created. I hope that makes sence. A couple of things; Braque said that he couldn't paint a woman in all her natural beauty, because he didn't have the skill, no one has and that he must therefore create a new kind of beauty, the kind that appears to him, in terms of form, volume - all the mechanics of a painting.
Have a look at Picasso's portrait of Gurtrued Stein, it looks nothing like her, yet according to her, it was the only picture of her tht was always her.
Finally, I remember reading that every successful portrait has an element of characture in it.
I hope this is useful and not too rambling.
Anthony.