Here's a post I accidentally put in the screenplay-comparison thread, but it really belongs here.
Story Ennis is not only less homophobic than Movie Ennis, he's also more chatty. Movie Ennis is downright shy and taciturn, whereas Story Ennis talks about the same amount as Jack (another reason the story characters seem less distinguishable).
I always figure Movie Ennis is quiet not only because that's his inherent nature, but because of his childhood experiences. The Earl episode was just the tip of the iceberg. If Ennis could even consider the possibility that his dad, someone he seems to have respected, tortured someone to death for being gay, we can assume that his entire childhood was pretty terrifying and confusing and shame-filled and awful. And we can figure that the fear of revealing that one big part of his nature scared him into keeping ALL of himself hidden.
Judging by personality, talkative Story Ennis would appear not to have had that same kind of a repressed childhood. And that matches what seems to be the main reason for Story Ennis to reject Jack's offer: because of Earl, he knows that living with another man would be dangerous. It's more a pragmatic decision based on an isolated trauma, than the result of inner conflict built over years of fear. The larger implications of how the Earl incident might have affected his youth, though still there if you think about it, are not reflected in Ennis' personality.
BTW, I keep wanting to start a whole thread about Ennis and the Earl incident. So often I see people say Ennis behaved the way he did as if it's the result of that one horrifying experience. Whereas in fact that's just a story that lends itself to telling, both by Ennis and the film/story, but suggests much more than that. "For all I know, he done the job" is, for me, the key line.