Hi Cara,
I assume you are speaking of the book "Brokeback Mountain Story to Screenplay" which also includes Annie Proulx's original story, and three essays by Proulx, Ossana, and McMurtry and finishes up with a Cast and Credits Listing.
If so, the screenplay portion begins with a note: For Publication October 2005
Filming was done over 4 months beginning in late Spring of 2004.
And although I can't give you any examples right now, I know that the dialog written in it, does vary some from the actual film dialog in some places.
Keep in mind that a screenplay is written, usually, before any filming starts, but undergoes revisions even while the filming starts.
And also keep in mind that the director films scenes in any order he chooses with some intentional editing in mind, but then ultimately, combines these many many filmed scenes in the best way he wants at the time of editing. So in fact often SOME of what is filmed, ie, some of a scene, or even ALL of a filmed scene is later determined not to work properly within the framework of the overall film as editing progresses. Lee has spoken in particular of the Murder scene, which is much longer than the few fleeting images we see, and how he had intended to insert some of that in the scene with Jack's parents, but ultimately discarded it all from that point in the film as it did not enhance and in fact ruined the intended message and mood there.
So, what I am saying in my usual wordy way, is.....the screenplay is a work in progress UNTIL the film is finished, and even then, some of what got left out in image editing or ad-libbed into a scene still isn't reflected in the published screenplay.
Hey why didn't I just say: It is NOT a transcript of the film!
Pretty close...but not.
And as for stage directions etc, it's not a play, so I would bet that what we have for stage directions in this published version is more or less only to make sense of the screenplay we have, and likely was NOT actual directions for any of the crew. I'm sure those were spelled out in various specific versions.
Hope this helps and feel free to correct or add.
Jack in Maine