This should be a really boring movie. The outcome is a foregone conclusion: we know what’s going to happen; we know he’s going to survive the climb. It happened in the past, and it made big headlines. And yet, I was sweating bullets the entire time I watched him go up El Capitain, with no ropes, and no margin for error. My palms were basically dripping with sweat… it is so incredibly tense to watch this kind of feat, in full knowledge that it is in no way fake, and that his life is on the (lack of) line the entire time. Much of this stress is brought about by watching what his friends and fellow climbers, who were there to document the achievement on camera, lived through as they watched the drama unfold real-time.
As is discussed in scenes where Alex undergoes studies in an MRI machine, he basically has a tiny amygdala, leading to low levels of anxiety/stress in these situations. And a huge pair of balls.