Heath Ledger's screen time. (spoilers)
I recently read about adapting 'Brokeback Mountain' from page to screen. In it, one of the screenwriters says that seeing Ledger's performance in this made them say that he was perfect for Ennis.
So that, plus him being on the cover of the DVD, made me think he was going to be a major character in the film, so I was shocked when he died so early on.
That's quite an impression to make in such a short amount of screen time.