Some people become adept at completely hiding their natural reaction to a situation. Some may call this lying, or just control.
Neither of these describes when it is done swiftly in the moment, and done with such skill that most observers will not see the 'glimmer' of the natural reaction.
I think this is a form of lying, psychoanalytically-speaking, but I do not think malice is an inherent condition here - sometimes a leader or a parent will swiftly hide concern from their children or those who look to them for guidance. This latter case is not evil.
I would think that 'deft' is related, but am unsure if the element of concealment is an inherent quality of deftness. I am speaking to the swift-covering of the natural response, and want a word identifying that a natural reaction began, evanescently, and was deftly replaced by another visage.