Catchlight

Every light in front of a person leaves a mark in their eyes, and its shape gives the source away: a rectangle for a softbox, a ring for a ring light, a hard dot for a bare bulb, a soft wedge for a window. Without one, eyes read as flat and dead, and a portrait looks subtly wrong for reasons most viewers could never name. Getting one is easy. Bring the key light round towards the front and lift it slightly, or put a reflector low to bounce light back up. Check before recording, because no grade will rescue dark, empty eyes.