Model shots are used differently than actors headshots but basically the agency will look at a series of headshots and select those that have the general "look" they're after.
Actors and models have auditions and the headshots are then used only to give a visual cue as to the person doing the audition. Very few decisions are ever made from just a headshot.
98% of the headshots I see (we have a brazillion of them) emphasize the eyes nose mouth looking up at the camera with paramount lighting.
Tom
Box Office Manager
Stage West Theater