Facial expressions have many meanings depending on contexts and relationships. Take the smile for example, it has many functions

Facial expressions carry meanings determined by contexts and relationships. For instance, the smile, which is typically an expression of pleasure, has many functions. A woman’ s smile at a policeman who is about to give her a ticket does not carry the same meaning as the smile she gives to a young child. A smile may show affection, convey politeness, or disguise true feeling. Pain is conveyed by a grimace, which also signifies disgust or disapproval. Surprise, shock, or disbelief can be shown by raising one’ s eyebrows. A wink given to a friend may mean " you and I have a secret" or "I’m just kidding". Between a man and a woman, a wink can be flirtatious. Our faces easily reveal emotions and attitudes.
    The degree of facial expressiveness also varies among individuals and cultures. The fact that members of one culture do not express their emotions as openly as members of another does not mean they do not experience emotions. Rather, there are cultural restrains on the amount of nonverbal expressiveness permitted. Given individual differences, it is difficult to make generalizations about a cultural style of communication. Americans express themselves facially in varying degrees. People from certain ethnic backgrounds in the United States may use their hands, bodies, and faces more than other Americans. There are no fixed rules, although it is considered negative or suspicious to have a "deadpan" expression or a "poker-face". Some people can be "read like a book" , others are difficult to read.