In the book, when the description of Jack's body language is given, Jack is not in the apartment at all and he is standing side by side with Ennis when Alma opens the door wider the 2nd time.
Ennis did not go down to the ground level when Jack arrived; Jack ran up the stairs, two and two, to the upstairs landing where Ennis was standing.
In the book, there is no detail as to what happened when they went back to Ennis's place to get his stuff and they go camping for a few days. Ennis calls Alma from the rented room in the Motel Siesta to tell her what he is going to do and then there is a triple-line spacing after that paragraph to show that what is in the next paragraph happens later.
The next paragraph has Alma working for the Riverton grocer. After Ennis and Alma moved to Riverton, he worked on the highway on weekdays and on weekends, he worked at a ranch to pay for the keep of his horses. (The movie has Ennis working for the highway department before they move to town.)
I really believe that Jack Twist really did not want to meet Alma in the first place. That's why he was so nervous when he saw her after she opened the door a 2nd time. If she had not opened the door the 2nd time, I just think that Jack would have been told to go back down to his truck and wait for Ennis, or Ennis might have just opened the door and got his hat and said, "Alma, see you later."
The movie has Ennis drinking several longnecks (that's beer in bottles) during the day before Jack arrives. But, according to Annie Proulx, there is no consumption of alcohol in the apartment that day. And, even after the guys get a bottle on the way to the motel, they are making out in the motel room 20 minutes after they leave Alma behind.