Mark Wahlberg is seen here with a marine at Camp Leatherneck in Afghanistan. This photo provides a great illustration in contrasting smiles. The female marine standing next to Mr. Wahlberg, is wearing a moderate sincere smile, her top teeth are exposed (while her bottom teeth aren't) and her eyes are partially closed. We cannot see her forehead, but there is a very high probability it is relaxed.
Mr. Wahlberg is looking into the sunlight, and while his degree of eyelid closure is consistent with a sincere smile, he's squinting actively, using his inner eyelid muscles - rather than the secondary closure we see with the "bunching up" of outer eyelid muscles. Mr. Wahlberg's respect for the marines and his sincerity is not in question - he visited Afghanistan of his own accord - and this particular expression we see on his face is a reaction to the bright sunlight. It is however, identical to forced smiles which we often see when there is no bright light present - sunlight or otherwise. His nostrils are dilated and his mid-forehead is contracted. His mid-face is also significantly tightened. All of these qualities are highly consistent with the emotion of disgust. Yet we see this type of false smile exhibited many times every day. Do you see sincerity where there is none?