Everything I paint is a portrait, whatever the subject.
I'm an odd portrait painter in that I'm not just interested in human faces. I consider almost all of my paintings to be portraits.
When I paint a portrait I want to know more than just the looks of the person. I want to know how they live and what their feelings are... It then becomes more than just physiognomy, but the feel of the person.
The danger, I find, is that you can become too formulaic, like some commissioned portrait painters who develop a methodology.