The eyes are "windows to the heart." In most people, they reveal a person's intentions. A sharp inhalation, deep squint, hardening of the face, or barring of the teeth indicate a immanent attack. Darting eyes indicate fear, looking for an escape, or looking for help or a weapon. Upon observing any of these indicators, do freeze your gaze on his or her eyes. While the eyes may communicate intended actions, keep watching the triangle for indications of any action.
Be leery of the "cold stare." The cold stare is one that shows no emotion and thus hides intentions. Be especially leery when the cold stare begins to look through you or to your side. If you are familiar with dogs, you know that this look means an attack is imminent.
I first started my martial arts training in karate. There was a senior color belt in the class who had black, cold eyes. If you ever looked at his face while sparring him, you would be psyched out and your confidence would be drained.
While in the Navy, I once had a commanding officer, Captain Weir, who had black eyes that seldom blinked. When he stared at you, it sent cold chills down your spine. He could elicit fear simply by looking at you.