Usually, we dont cry at our desks at work, run down the halls in happiness or punch collegues who frustrate us. We live in a culture that doesnt encourage tremendous emotional displays, especially in the work space. The rules of expression in a culture are called cultural display rules, and they vary depending on the emotion, the person expressing it and the situation.
Because we dont always feel internally what the cultural display rules dictate, our nonverbal behaviour can reveal feelings that we wish to conceal. We may not be actually sad at a funeral or happy at a wedding, but the nonverbal communication can reveal how we really feel.