Filming & Production
Key light
Also called: key
Everything else in a lighting setup is defined against the key. A fill softens the shadow the key casts, a back or hair light separates the subject from the background, and a practical adds interest. Move the key and the whole shot changes; adjust the others and you are making refinements.
Two properties do most of the work. Size relative to the subject controls how hard or soft the shadow edge is, so a large diffused source placed close gives the flattering look most talking-head channels want. Position controls the mood, with a key near the lens flattening the face and a key off to one side carving it up. A window makes a perfectly good key and costs nothing.