My husband, usually a cool customer, watched Free Solo from behind his fingers, sometimes jumping up from the sofa and backing away from the TV. Audiences at Imax showings have behaved the same way, rising to their feet, clenching their sweaty fists as they watch  Alex Honnold, a 33-year-old rock climber from Sacramento, make his way up El Capitan, 2,700 vertical feet of granite in Yosemite National Park.