Let’s all talk about the fact that Cas is EATING the popcorn.
Cas has only eaten once up to this point, and that was only because his vessel was affected by Famine, which was beyond his control. He does not need to eat and does not think to eat. And yet, in this scene the popcorn is sitting closest to him, the only person in the room who does not eat.
It’s unlikely that he made the popcorn because, let’s face it, he’d probably set something on fire doing it. So he probably didn’t make it and carry it to the table and happen to set it down there, which means Sam or Dean did. Given that Sam is messing with the projector, we can reason that Dean most likely made the popcorn and brought it to the table. And yet, instead of putting it close to himself, or in the center where they can all reach, or between himself and Sam (you know, the people who EAT), he put it in front of Cas.
And Cas ate it.
Let’s also think about the fact that in the same episode, Dean was attempting to feed Sam because he was injured. Cas is also injured, and Dean, despite what he says, still wants Cas to “get better”.
His instinct, whether he’s aware of it or not, is to take care of Cas because he cares. And Cas accepted this gesture even though he does not need it, because it is from Dean. Without words, they give and take, and in that act shows their profound bond is still there, and still strong.