The expression displayed by President Donald Trump in these two images were almost never seen on Candidate Trump.
The primary emotions he's projecting in both of these photos are that of bitterness and regret. Note the corners of Trump's mouth are down-turned. This is sometimes referred to as a "Bitter-Smile" - very ironically to most people, for it resembles nothing of joy or happiness.
The President is also displaying a significant element of frustration as evidenced by the clenching of the jaw, dimpling of the front of the chin, and the bulging forward of tissue just beneath his lower lip.
In real-time, while people are displaying this expression, virtually everyone believes they are "pushing out" a slight smile (e.g., forcing a smile). However, if by some chance they could momentarily "freeze" their face - and immediately look in a mirror, they'd be very surprised to find their expressions' are quite far from what they believed them to be. This is a universally human phenomenon - for as we deviate further from our "emotional baseline", our objectivity of the true configuration our faces' and bodies' becomes quite distorted. We also become increasingly inaccurate as to the correct interpretation of what other peoples' body language is "saying" (as well as what the simultaneous verbal language and vocal qualities are conveying).
Further thin-slicing the emotions in each of these pictures reveals another distinction. In the image above, Donald Trump's eyebrows are more drawn together and vectored downward - while in the image below they are neutral or perhaps slightly elevated (more so on his left). This subtlety is significant for a higher level of frustration (and shorter temper) in the moment above - and a surrendered-resignation in the lower image.
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