What changes occur through the movie! Yet they happen so subtly, so gradually, that we are lulled into thinking nothing much is happening. In fact, we may even come away thinking, why couldn't Jack be satisfied over time with the status quo as Ennis laid it out??
But let's look at some of the details more closely. Jack started out being at large and in charge. He was the first to light up his cigarette after leaving Aguirre's trailer, using his lighter rather than a down-scale match. He smoked whole cigarettes, whereas Ennis had to put his cig out partway through and save it for later. Jack had two beer bottles in front of him at the bar, as opposed to Ennis's one. But as early as the bar scene, Ennis was already borrowing Jack's lighter. Later, Jack was out in the cold on the mountain, forbidden by Aguirre from having a fire, looking down on Ennis's "night fire." Jack was also sharing his whiskey with Ennis. In fact, Annie Proulx writes that they had some drinks that were "on shares." (My theory is that Jack brought the whiskey stash up there himself because the employer would be unlikely to provide it and he had learned the year before when surrounded by stinking electrocuted sheep's carcasses how important it was to have whiskey.)
Ennis got bolder and bolder. In fact, when Jack offered him a canteen, he said "Got whiskey or somethun?" Ennis was clearly upset about having to ask for whiskey, so Jack learned to provide it without having to be asked. In fact, Ennis didn't like touching, so instead of passing him the bottle, Jack poured the whiskey in Ennis's coffee cup. Later after the boys were both soused, Jack got bolder and passed the bottle, but he still grasped the neck, giving Ennis plenty of room to grab the bottle without touching his hand.