A portrait photographer tries to capture someone’s personality in a photo. Sounds cliché, I know, but there we are. Unless it’s for a passport photo or an ID, a portrait is more interesting if it tells us something about the person in it. Watching people in public places, if you can pull it off without being rude, is a great way to practice your skill of studying a person and finding an expression or a gesture that tells you how they feel, what they might be thinking, or something about their likes and dislikes.
Changes in expression happen at split seconds, and it pays to anticipate a facial expression of the subject the split second before you press the shutter. One of the best observation skills I’ve learned without my camera in front of my face is learning to anticipate when something will make someone smile, or frown, or do something interesting with their face.
Light distracts me to no end. I’ll be walking somewhere or driving somewhere, and all of a sudden in the middle of a conversation, I’ll yell “Ah! Look at that light!” simply because it’s so beautiful. And scenarios would form in my mind of the kinds of beautiful portraits I could make using that light.

Lighting changes the mood of a portrait, so it pays to know in which light you can achieve a certain mood in a portrait.