The eyes are part of "what lets you know". People who are lying cannot maintain eye contact, rather they occasionally glance and their eyes dart around. If someone is genuinely interested in a product I offer them their pupils will visibly expand, and the reverse if not.
Sven Goran Eriksson never looks at the camera when interviewed post-match, the reason being he is not in the least bit interested in addressing the watching fans.
Some police officers are able to remain completely expressionless when interviewing suspects. This is extremely difficult to do, and requires training.