You're right about that compartmentalizing thing, d. So, someone asked me a few pages back how Jack changes his behavior when he's with Ennis. I'll just mention a few examples. First scene, when Jack starts out to walk up and introduce himself to Ennis in his straightforward way, Ennis reacts by lowering his hat brim. So Jack comes to an abrupt halt and, after thinking about it, retreats to slink by his truck. He even gives Ennis more space by turning his back to him to shave, peeking thru the rearview at him.
Next we see Jack being his exuberant self, strutting in front to the bar, drinking two beers to Ennis's one, drawing Ennis out in conversation and, later, selecting the most high-spirited horse for his mount. But Ennis warns him to tone it down, saying the horse has a "low startle point" (was he talking about himself?) Making camp, Jack splashes water and whacks with an ax, while Ennis plods along with a saw, and laboriously arranges rocks in a fire ring. Working together, they get accustomed to each other's style and temper their behavior to fit the other's. Two weeks later, Jack remarks on Ennis actually being conversational, as Ennis even makes eye contact and says "what?" to invite Jack to take a risk with him.
But Jack still doesn't behave the way he normally would. When Ennis walks away to strip and bathe, he doesn't look. And there's the aftermath of the bear scene that Mel mentions. When Jack's horse continues to be high-spirited, Jack is embarassed. Even though he lets his guard down, Jack can't just be himself around Ennis. If he did, then the events of the last day on the mountain wouldn't be plausible and Jack would have never driven away and left Ennis walking down the middle of the road. Also, he wouldn't have had to steal Ennis's shirt. It just wouldn't have made sense.