I don’t do much portrait photography. Most of my people shots are casual and unposed. The majority are of family, and it seems we are often wearing sunglasses.
But if you want to photograph faces the best way, catchlight is an important element to be aware of. A catchlight is a highlight caused by reflected light on the surface of the eye (as opposed to red eye, which is light reflected from the retina). Eyes without catchlights can look flat and dull, while catchlights add sparks that can seem to animate faces.
They come in all sizes and shapes, and it’s possible to have more than one. Studio photographers often use light reflectors to make sure they get catchlights in their portraits. If they get more than one they are likely to edit one out in post processing. Traditionally, the best positions were thought to be ten or eleven and one or two o’clock. Some photographers will even add these highlights in post if they failed to capture them in making the photo.
In outdoor settings the best way to get catchlights is to focus on a subject who is in the shade facing toward the sun. (Some photographers carry pocket reflectors, which they usually position below the subject’s chin—these would be most useful in posed situations, and I’ve never tried them.) If the subject is looking at you as you take the photo, try to have the sun at your back. Just be aware that you might end up with your own reflection in the subject’s eyes.