A smile is the easiest of the facial expression to reproduce voluntarily. Although there could be mistakes made in the timing of unfelt smile: too quick or too slow. Evident mistakes can be spot too soon before or too long after the word or phrase they should accompany.
Negative emotions are harder to recreate and people fail to falsify them. People find difficult to move, voluntarily, those muscles required to falsify distress or fear.
Anger and disgust are a bit easier to reproduce when they are not felt.