"For my part, each time I appraise a person’s face, I am inspired—you might even say carried away—not by his or her outer aspect, which is often trivial, but by the characterization it can be given on canvas. That is why I am accused sometimes of having my portraits look like caricatures."
If you're trying to learn about portraits by questioning a portrait artist, you're barking up the wrong tree. Pay attention to cartoons, animation, illustration, dance, acting, anatomy, the anthropomorphism of animals, fashion, literature*, graphic design and psychology. That will teach you a lot more about portraiture.
*Especially recommended for portrait-reckoning: anything by Jane Austen or the Brontë sisters; The Hours by Michael Cunningham; any story by Flannery O'Connor.