How can you hold someone's attention on a photograph? What about making barriers to stop them from leaving?
Photographs are not often studied for long periods of time by the casual viewer, but I think if you can hold someone's attention for 5 seconds rather than two, the image is somewhat victorious.
Many things can grab the viewer's attention - story, metaphor, gore, et-cetera - but in this post I'd like to talk about figuratively bouncing the viewer's eye back into the picture as it is trying to leave.
Light sources from camera left create shadows on the right so that the eye reads across the image and the shadow returns the eye to the left side.
If a person faces left, the viewer follows their gaze back across the picture. If they are facing right it is almost like the viewer can skim across the picture and has permission to exit the right side of the frame.
Bouncing the eye back into the picture is also the purpose of a matte in a frame - that is why wide mattes (relative to the picture) are desirable rather than simply filling the frame just with the picture. It provides a wasteland the viewer must cross to exit, and they may decide to return to the picture by force.
I find this theme interesting because it implies the direction a subject faces, and the position of the key light are not at the photographer's whim, but are best kept constant to capture the attention of a viewer. Of course, the converse is also useful - you may want a rapid flow of the viewer’s eye from the left and out over the right side of the frame.
There are plenty of examples dating back to renaissance paining. The lighting on the Mona Lisa is from the left and her eyes take you back left. Read across Caravaggio's “Calling of St Matthew” and you’ll find Jesus pointing you back left from the shadows. Those painters didn’t leave anything to chance - every minutia of the picture is intentional.