Heath Ledger and Jake Gyllenhaal were perfect as Ennis and Jack. Now that they have turned in their immortal performances, they have indelibly marked themselves in our brains, and, as Annie Proulx noted, they knew the characters better than she did. But if you read the descriptions of Ennis and Jack in the story, they are quite different than Heath and Jake. If you were casting the movie, who would you have picked?
Reading the story, here's who I would have selected to play Ennis. He's a songwriter turned actor, with the required high-arched nose, caliper legs, and body made for fighting:
http://www.imdb.com/name/nm0005164/