Voluntary and involuntary expression
In advertising fake smiles are used to make us desire something. We think the person — actually a model — is really enjoying the food or getting their wash cleaner than ever. The portrait photographer asks us to say “cheese” so we will smile, i.e., looking like we are smiling.
Paul Ekman's research observed a close connection between facial expressions of emotions and changes in the body.
When you intentionally make a facial expression, you begin to have the changes in your physiology that accompany the emotion
The face, it turns out, is not simply a means of display, but also a means of activating emotion.