DEAN Sam, listen to me. I made you a promise in that church. You and me, come whatever. Well, hell, if this ain't whatever... But you got to let me in, man. You got to let me help. There ain't no me if there ain't no you.
SAM What do I do?
DEAN Is that a yes?
SAM Yes.
Sam saying "yes" to Dean (who's really Gadreel)... it's interesting the way it's worded "But you got to let me in," which is what possession is..