When filming people, the goal is to capture their emotional state. And the only way to do that is to get a shot of the person's face filling up a significant portion of the screen. Professionals call these shots "close-ups." Customarily, a close-up leaves a little bit of room above the person's head (not too much!) and includes the entire face and shoulders down to about the armpit.

Why include the shoulders? Because if the shoulders are cropped out, all that remains is a floating "disembodied" head. It's psychologically disturbing—that's what early filmmakers discovered. Showing the shoulders implies that the subject actually has a body—even if we don't see it.