Most of those who see this photo have no problem seeing that Sam Rockwell is angry. The most obvious signals of this emotion are his medial (inner) brows are lowered and are pulled together. The currugator muscle is also contracted giving those characteristic vertical lines just above the nose in the central lower forehead. Interestingly, this can also be a facial signal for moderate to intense concentration. So be careful to not mistake the thinker for the angered.
Looking for the less obvious, we find that Sam's eye lids are somewhat closed. This can also be a sign of subtle to severe anger. Here we see a subtle version of Clint Eastwood.
But the reason I chose to use this photo is because it contains a gem of a body language signal in another part of the face. Look at the mouth. You see a fine example of the display of lower teeth. No upper teeth are visible. When only the lower teeth are displayed and those upper 16 are hidden - think anger! I don't mean to imply that the upper teeth must be hidden for anger to be present. There are certainly classic mouth displays of anger where both the upper and lower teeth are visible. But you should endeavor to learn all the nuance and variation of nonverbal communication. If Mr. Rockwell's eyes were relaxed, but his mouth was unchanged - this should still send an anger signal to your gray matter. Being aware of the subtle signs of emotion in others will put you a mile ahead of everyone else. Knowing what people are thinking and feeling is a very real kind of magic.