I missed the equivalent of this thread on the IMDb board - thanks for starting it here, Amanda.
I haven't traipsed through all of the previous posts, so the following might be repeating something
someone else has said.
There's a wonderful, easily missed moment in the first scene just before Agurre opens the trailer door to snap the first spoken line in the film, "If you pair of deuces....scrawny asses in here, pronto."
Jack had been shaving before Aguirre drove up [discretely checking out Ennis in the side mirror] and had hurriedly wiped away the remnants of shaving cream off of his face as Aguirre's car barelled up and the latter had stormed into the trailer.
As the two boys watch with baited breath to see if and when Aguirre will deign to come back out and call them in, the viewer can just barely discern Jack in a long shot from the rear nervously wiping his face again as Aguirre is seen walking back and forth in the trailer and Ennis has his back to Jack. I like to think that Jack is furtively wiping his face again not for the sake of impressing Aguirre, but this time to make sure he looks his best for Ennis.
It's not mentioned in any of the versions of the three screenplays I have, so credit must be given to Ang Lee and Jake Gyllenhaal for this detail in a film with countless such subtle and meaningful touches.
Casey Cornelius