Much like with the rest of the body, for that matter. Most people rely on verbal communication a lot, but general and subtle body language (from your forehead all the way to your toes) tells more then people realize.
Even a split second twitch of a random muscle can give away volumes. Interrogators are experts in ready body language, for example. Depending on how unconsciously expressive someone is in their body language, they can tell whether someone is lying or telling the truth.
Facial muscles in particular (including eyes and direct surrounding area) are mental tattletales.