This is amazing!

I'm impressed with how you managed Cas's expression. It is so un-canon to have him smile like that and still, it feels like it's really him! If only!
Also I think that the reason I like Cas more in this piece is that you drew him in a slightly more illustrative style and it feels more personal, like it's got a part of you in it, your touch. It's difficult for an artist working with real-life models to find the middle ground between resemblance with reality and your own personal universe and I think you've really succeeded there. I think it takes a lot of working with a character to get to this point and it shows here.
If I had to say one negative thing about him I'd say his hand is a little big proportionally to is face but that's anatomy for you, I think we all struggle with that...

Dean's face on the other hand is more realistic thus less interesting in my point of view. Your technique in that regard is really good though.

As for the general ambiance, I'm not a fan of the blurry style, it tends to confuse my eye and prevent it from really focusing on the picture. It's also often used to make up for a lack of technique or precision (which I'm not saying is the case here) so I tend to look at it the bad way. It is relevant sometimes though, here I'm not sure, but I won't pretend I would have know better.

I'm a little disturbed by the size and the font of your signature too, but that's always a dilemma. If you want it to be noticeable and to advertise for your work it's probably gonna get in the way of your painting.

Anyway, you stole my heart with that one!
I wish I could read the fic now