From Paul Ekman, Telling Lies: Clues to Deceit in the Marketplace, Politics, and Marriage. NY: W.W. Norton, 2009, p. 33—
The best way to conceal strong emotions is with a mask. Covering the face or part of it with one's hand or turning away from the person one is talking to usually can't be done without giving the lie away. The best mask is a false emotion. It not only misleads, but it is the best camouflage. It is terribly hard to keep the face impassive or the hands inactive when an emotion is felt strongly. Looking unemotional, cool, or neutral is the hardest appearance to maintain when emotions are felt. It is much easier to put on a pose, to stop or counter with another set of actions those actions that are expressions of the felt emotion.