"Pay attention to smiles in videos. People will be able to determine a fake smile versus a real one better in a video than in a photo. If they don’t think the smile is real, they’re less likely to trust you."
"It is possible to fake a smile and to fake a crinkly-eyed smile, but it is easier to fake a smile in a static picture than on a video."
"People can tell whether a smile is real or not by looking for conflicting emotions. They are looking at many parts of the face, not just the eyes."
"If a smile looks real, it will engage the viewer and build trust."

Weinschenk, S., PH.D. (2011). 100 Things Every Designer Needs to Know About People [Kindle]. pg.162