In her essay "Getting Movied," Annie addresses this very subject. She lists a bunch of realistic details from the movie: Ennis dirty fingernails in a love scene, the old ENTERING WYOMING highway sign, the splotch of nail polish on Lureen's finger, Mrs Newsome's Texas hair, the speckled enamel coffee pot ... then says,
" ... all accumulate and convince us of the truth of the story. People may doubt that young men fall in love up on the snowy heights, but no one disbelieves the speckled coffeepot, and if the coffeepot is true, so is the other."
(Thanks to F-R for her generous loan of "Story to Screenplay"!)