You want to know the truth? Then don't mimic!

We usually feel that expressing empathy by mimicking another person's face and body movements facilitates our understanding of their true emotions. Not so, apparently, if they are lying. Stel et al. have done experiments with two interacting people as follows:

...targets either lied or told the truth, while observers mimicked or did not mimic the targets' facial and behavioral movements. Detection of deception was measured directly by observers' judgments of the extent to which they thought the targets were telling the truth and indirectly by observers' assessment of targets' emotions. The results demonstrated that nonmimickers were more accurate than mimickers in their estimations of targets' truthfulness and of targets' experienced emotions. The results contradict the view that mimicry facilitates the understanding of people's felt emotions. In the case of deceptive messages, mimicry hinders this emotional understanding.