Shock, first off, closely followed by a flash of anger, all quickly covered up by Dean's usual against-the-wall, let's-get-this-shit-done glower, mouth pulled into a thin line and a machete in his hand. A machete he plans to use on the leviathan and not Castiel. Probably.

The leviathan bangs on the door, not really trying to break through, mostly just trying to make them jump. It kind of works, Dean's hand twitches, and he shoots a look at Castiel.

"Soon as this thing's dead, we're talking, you and me."