I was reading about the lifelong friendship he had with Jon Voight, which began during the filming of Deliverance. Burt enjoyed teasing Voight's "method" acting. There was a scene in the movie where both characters were supposed to be exhausted and out of breath. Before the cameras started rolling, Voight sprinted up and down the road a few times, dropped to the ground and did twenty push-ups. When he stood up, he was panting, dripping with sweat.
Burt, who sat in a chair watching Voight, got up, took a water bottle and spritzed himself, and simply started panting, for the same effect.