If you are an avid fan of action films and classics like Batman, you might be inclined to try and look for facial features to identify the silhouette that approaches you in the dark.
But you really should be looking at your opponent’s head level (not at the eyes trying to see them flinch,) The idea is that you do not want the person's both arms and legs escape your vision.
A silhouette is approaching at the dark, is it an instinct to try and identify the face, or perceive it as a threat and learn to be alert? Are we going to look for the arms for weapons? Are we going to wait for the point that even if we notice a surprise weapon it would be too late for us to react?
Our eyes serve as distance gauge and a tool for perception of the predicted scenario. We need to see what to expect, we need to see what we catch, stop, or where we aim. We can never see our own hands in complete motion because it would slow it down. Do you think a golf player or a batter can see his complete swing at the same time he is swinging?
So you basically try to keep your head facing centered to where the opponent is, keep your eyes focused on the center, on the vertical line of the opponent's body at your eye level.