I think actors like Jensen and Misha are really attuned to their characters and bring a lot more natural craft to their work, so they can take the writing or the direction and make something new and personal out of it. Dean could have been such a one-dimensional character if Jensen hadn't breathed life into him, the same with Castiel.

Jared is probably the sort of actor who needs strong writing and good, clear direction, which this team hasn't been able to offer, especially where the character of Sam is concerned. They've been floundering, so he's been floundering. I was already starting to lose touch with Sam in S3, where he so often seemed self-pitying rather than genuinely concerned with trying to save Dean. S4 was a hopeless mess for both Sam and Jared; S5 and especially S6 have done nothing to get either the character or the actor back on track.