SO--I would now say that my (ahem!) continued research into FNIT has been critically informed by the aforementioned statements made by Mr. Ledger. I conclude that his portrayal of Ennis Del Mar in this scene, and all prior and subsequent scenes, is characterized by the highest possible degree of veracity, truthful adherence to the nature and motivations of the character as conceived by Annie Proulx, Larry McMurtry, and Diana Ossana.
In other words, Ennis ain't fakin'! He and Jack are the real thing here, in the tent, exploring their newfound love--and lust--for one another. Yeah!! This has to be my all-time favorite thread. :D