Dean's aware of the music first, that Nat King Cole record he hasn't listened to all the way through yet, some scratchy forties song that sounds kind of nice even if he would never admit it to Sam. It's when he notices Cas that he realizes it's not real.
Actually, he can't really see Cas, because he's looking over the angel's shoulder, arms around him. They're dancing, sort of, though he knows he doesn't really know how, and doubts Cas does either. It's just a kind of swaying and stepping to the music, and though Dean doesn't remember how it started, he doesn't mind, just lets himself slip into it.
"I'm dreaming," he says as a new song starts up, like he isn't already sure. The room is too bright, the dancing is a bit out of the ordinary, and besides, there isn't a chance in hell Cas could be here.
"Does it make a difference?" Cas says against his ear, sounding much lighter and more at peace than Dean has heard him in years. Dean doesn't say anything, just reaches up to grip the back of the trench coat tighter, because even if it isn't real it feels real, and that's as close as he's ever going to get now. They dance, and it doesn't matter that they've never danced before or that Dean's going to wake up or that Cas isn't real, for some precious minutes it's enough.
Then Dean says, "I'm sorry," his voice betraying him on the last syllable. He's still leaning against Castiel's shoulder and looking over it, but now it's because he can't bear to meet Cas's eyes.
And then the same old slideshow he's been torturing himself with for the past month slams before Dean's eyes, Cas telling him he'd kill himself if he went back to Heaven, Samandriel's vacant, dead vessel and Castiel's vacant, dead eyes, announcing his return to Heaven. "For letting you die alone," he says, because he knows what it's like to die but he's never had to die alone like he's sure Cas has. He can barely get the words out, and neither notices that Nat King Cole has stopped singing and the record's spinning listlessly in the corner. Neither notices that they've stopped dancing, that they're just hanging onto each other in the middle of the room.
"What makes you think I died?" Cas says finally, still in that light tone, but there's a catch to it now. He feels warm and alive and Dean just wants to live in this dream forever.
"Because I know how this works," Dean says, dejected, hands moving again for a hold on Cas's trench coat as he feels himself dropping into some bottomless trench he'll never climb out of. "You leave, and then you come back, and then you leave me again. And I can't-" He shuts his mouth. The record player issues a string of static.
"I'll come back," says Cas, and this time his voice isn't light at all, it's serious and solemn and more real than anything else in this surreal scene. "I can't right now. I'm trying but I can't. I will, though." He starts to shift again, dances again though there's no music. "I promise."
Dean presses his face against Cas's shoulder, moves with him, because if Cas can pretend for a little bit longer than so can he. "I don't deserve dreams like this," he mumbles into the coat, thinking of his usual Hell nightmares and Purgatory flashbacks compared to this.
He thinks he hears Cas above him say, "What makes you think this was for you?" but it's quiet and low like he's saying it to himself, and before Dean can reply he's awake again, alone in his bed.