Understand the ways in which one can create light in the eyes.
Eye lighting can come from direct lighting, with filters designed to blend the light invisibly with the rest of the face, or the light can be gentle reflections in the eyes, or a hard point - as a cinematographer you need to know how to control the eye light with regards to size, shape and intensity. Eye lighting will, inevitably, be a major influence in creating the portrait.
“The Eye light provides a limited area of fill or highlight when the key or fill leave one or both eyes in darkness. So the eyesight is really a remedial light, it's what you use to remedy a problem. The eyesight is used when you can’t see enough life in the eyes and need to introduce a sparkle into the eye.”
Ian Murray, Cinematographer