I think I've always seen Ennis as stasis - he's so reluctant to move in physical or emotional space. He feels some kind of invisible force-field around him which he's afraid to touch in case it harms him, so he pulls himself into the little space where he's safe and cuts himslef off from possibilities, whether good or bad. Jack, on the other hand, is mobile, physically and emotionally - he also seems to me to have the attributes of a shape-shifter. He's open to whatever life offers, too, and is less concerned with protecting himself than Ennis is - fatally, as it turns out.