That was also my gut instinct. From what I can tell, Jared is the playful one and Jensen is the serious one. ("From what I can tell" means "what fic has told me" or "how they kinda seem in youtube videos from cons", lol). But I can see Sam being Eustace ("I JUST WISH THINGS WERE NORMAL") and Dean being Reepicheep (absolutely cheeky).

lolololomg! Jareep riding dragon!Jensen!! ANIMAL TRANSFORMATIONS YES PLEASE. I think Jensen would be perfect at those Eustacey glares! oh gosh dragon/human sexytimes! ... I have to say, I read a fic like that once. and it was really fun. XD