Jack and Ennis are poorly educated country boys, and Proulx helps us feel that in the way they speak. They don't use grammar well, and she often deliberately misspells words just to show us how simply and plainly they speak. Even when they talk about important things, like how they feel about each other, their words are nowhere near fancy. When Ennis describes to Jack what happened after they left Brokeback, he says, "I had gut cramps so bad I pulled over and tried to puke, thought I ate somethin bad at that place in Dubois. Took me about a year a figure out it was that I shouldn't a let you out a my sights" (68). Other characters use the same kind of tone, so we know that it's the culture, not just these two guys.
Proulx likes to get to the point, so she doesn't spend a lot of time discussing appearance. That means that the physical descriptions she does include really count.
Both Ennis and Jack share the look of poor country boys, used to the outdoor life and without two pennies to rub together. Jack "seemed fair enough with his curly hair and quick laugh, but for a small man he carried some weight in the haunch and his smile disclosed buckteeth, not pronounced enough to let him eat popcorn out of the neck of a jug, but noticeable. He was infatuated with the rodeo life and fastened his belt with a minor bull-riding buckle, but his boots were worn to the quick, holed beyond repair" (7). Ennis, "high-arched nose and narrow face, was scruffy and a little cave-chested, balanced a small torso on long, caliper legs, possessed a muscular and supple body made for the horse and for fighting" (8). So yeah, they're not exactly the tea-and-cozies types, which lets us know a lot about them very early on.
These are physical men, and everything they do reflects it. They move fast and suddenly, and nothing they do is particularly subtle. Their actions—and specifically the decisiveness of their actions—also say a lot about how they feel about their love. Ennis "ran full-throttle on all roads whether fence mending or money spending" (29), and Jack "took the stairs two by two" (47) when coming to see Ennis. Since these guys aren't much at using their words, it stands to reason that what they do would define them.