Liars' Poker-face Stronger at Mouth Than Brow

People sometimes intentionally deaden their facial expressions when they’re lying. They think they’ve completely eliminated them, but they’ve just reduced them. The mouth area is more easily controlled than the brow area (compare to fake smiles lacking the crinkling around the outside of the eyes). Also, the suppression may be non-specific - the liars will smile less in friendly interaction.

But the extra cognitive load in protecting a lie can be more detectible than physical signs. (liars use less details, or produce them less rapidly).