A smile that quickly dissolves as someone turns away can indicate falseness or an unusual emotional experience.

“the somewhat if not not very eerie sensation of somebody um smiling like a toothy smile and then it as they pivot away that smile just dissolving very quickly and um you know you don't have to be a neuroscientist or a psychologist to realize that like there was something quite false about that experience or that this person experiences um emotions like step functions on off on off which is not how most of us experience emotions most of us experience emotions with some pervasiveness”