He was taught at an early age never to show emotion in public. Nonetheless, the healthy human body seems unable to suppress emotion entirely. He might not cry, but his eyes may twitch. She might not sound nervous, but she may, literally, have cold feet. Emotional expression is not, clearly, limited to facial movement.
Vocal intonation is the other most common way to communicate strong feelings.
As these features vary, the emotional expression of the voice changes. The research problem of precisely how to vary these features to synthesize realistic intonation so far remains unanswered. Research to date has limited the speaker to a small number of sentences, and the results are still closely dependent on the particular words spoken. In this sense, affect recognition is similar to other recognition problems like speech recognition and lipreading.