Most of the time we are unconscious of what we are signalling with our eyes, but we can become aware of it and of the way it arises from our inner feelings. Hence, when acting, we can raise the emotional temperature by intensifying what we are doing with our eyes.
Just studying how long people look at each other tells us a lot about their emotional state. People who find it difficult to meet other people's gaze will appear shifty, while two people gazing long at each other presages either aggression or sexual attraction - the mouth will tell you which.
Here's an interesting exercise to try when you're walking along a busy street. Clock the eyes of everyone walking towards you, for just one second. Then tell yourself what their eyes tell you about their personality and emotional state. You will find that you can always find something, sometimes difficult to put into words, but always something about the way they are feeling and the kind of person they are.
The Eyes Have It.