Is Natalie Portman offering up a sincere smile here, or is she giving us a good social smile? One way to tell is the speed of onset and how quickly it fades. A sincere, felt smile begins and departs slower, while an insincere and social smiles tend to have a significantly quicker onset and are similarly fast to disappear. Of course, this is a still image, so we cannot use this method here. In the real world though, this dynamic is a reliable method to help detect sincerity.