People are two-faced
If you take a photo of a person and draw a dividing line down the centre of the face the two sides will often look different. Generally, the left side is more emotionally expressive. An emotion portrayed by an actor is more easily identified in the left half of the face than the right. The likely reason is that the right half of the brain ‘feels’ emotions more strongly than the relatively cold, logical left side and since the right side of the brain controls the muscles on the left side of the face the feelings are exhibited more on that side. The right side of the brain is also the reservoir of facial configurations and hence creates them more effectively in the face. You can try this with a photo of yourself; you might expect to see a stronger expression on the left side of your face, regardless of what that expression is (whether happy, sad or angry).