Just in this trailer, you can see how incredible an actor Phoenix is. He's doing
so much heavy-lifting and storytelling in his expressions. The trailer puts his intimate facial expressions at the forefront/as the selling point of the film which is quite remarkable given it's a DC comic-book film.
Like here, in the span of two seconds, we go from rejection/disengagement/self-loathing/bitterness to vulnerability/fear and finally to putting on a mask to cover up his problems with happiness. But his eyes are always reading something sadder and more sinister. Similar to the scene you're mentioning. Covering up intense and unbridled sadness with a superficial smile. His laughter at 1:05 is the most forced laughter of all time. And then that one at 1:11 where he's literally clawing his cheeks up to smile but there's absolutely nothing but hollow sadness beneath.
Shoutout to the way he runs as well. It's got so much energy and physicality to it. He looks so off-kilter, off-rhythm and desperate. Like a rabid animal trying to get free.