You'll be surprised to know that when asked to make a sincere smile on-demand, most A-list actors are not that skilled at it. This is not the case with Felicity Jones.
In this image, the Rogue One: A Star Wars Story (aka Rogue One) actor is displaying all the components of a fully expressed and sincere (aka Duchenne) smile:
1. Eyelids partially closed
2. Temporary furrow in each of her lower eyelids
3. A relaxed (non-contracted) forehead
4. Only the upper teeth visible (see below)
5. Upward vectored cheek muscle contraction
It's important to note that with a sincere, but suppressed smile - there may be no visible teeth at all (upper or lower). Also in the context of sincere laughter, lower teeth may be exposed - so on some occasions with a sincere smile the lower teeth may be seen when crescendoing to or decrescendoing from sincere laughter. However in the vast majority of scenarios, when the lower teeth are exposed, there is an element of insincerity present (perhaps a large one). This is not to say that people who are in any given occasion smiling insincerely are, as a whole, insincere people - it's just that IN THAT MOMENT there's insincerity being projected.
Ms. Jones has the additional quality of blushing easily. While a sincere smile does not require blushing - when it IS present, it's a additional and solid confirmation of sincerity.
Of all the components of a sincere smile, the changes in the eyelids are the most crucial - for without the eyelids being partially closed and without the dynamic furrow made in each of the lower eyelids (unfortunately often Photoshopped out) - the smile cannot be sincere. Thus when trying to smile - forget about what your mouth is doing and think ONLY of your eyes - your mouth will follow in kind. If the opposite is done (if you initiate the "smile" with your mouth) - the eyes rarely follow with sincerity.
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