A person's face can have degrees of opaqueness. A face can have different masks. Many people tend to wear a mask that is blank and unchanging. A kind of plastic image. But if a person is not trying to "mask" their feelings, and is feeling something, their face can be read easily, even though it would be impossible to explain how it is that their expression is being read. Like the eye can be a window to the soul, the face can be a page of thought, or feeling.
That a good actor can consciously do this is food for thought.