what a blast these last 24 hours have been. I still don’t know where my head is at. ;)
I talked a bit about Cas’ mindset yesterday, but didn’t have the info we got now. Still, I think to a big extent Cas seeing Dean as a demon must be especially painful, because the man he raised from perdition has become what he set out to save him from all those years ago. So I guess for Cas it might involve some thoughts of having failed. Not just failed Dean and Sam and and humanity, but as a person and that might also play into why he doesn’t really search for a way to get better but seemingly (as spoilers suggest) just lets his grace fade out. I personally would love it to pieces if we got some exploration of how far gone Dean was when Cas found him and maybe have Cas remember it when he sees Demon!Dean for the first time.
Long preface to the actual answer. I think the very first confrontation between Demon!Dean and Cas might happen in 10x03 as well, because Sam might call him for help. Really, I can only imagine how intense this first meeting will be judging from how intense the one between the brothers was and I fully expect Demon!Dean to aim where he’ll hurt Cas most and will probably go on to say something along the lines of “Ahh sweet, you’re burning out too bad it all could have gone such a different way…” But I am getting off-track again.
But yes, I assume that - let’s say from episode 6/7 to 10/11 it seems as if Demon!Dean - in case they shouldn’t have cured him by then and Dean is only still left with the mark - is a little more controlled only to then blow up in everybody’s faces and everything going really bad. And I think it will involve a strong Dean and Cas interaction. Cas might give the last bits of his grace to save Dean or try and kill him because he doesn’t want to see the earth burnt to the ground and the man he fell for in more than one way completely gone.
I could imagine though that Cas will act as if he’s got enough ~juice to take on Dean, but he doesn’t. And I expect Dean to be fully aware of the fact that he outmatches both Sam and Cas.
We have been speculating about a reverse crypt scene all last season and were assuming we might get one in the season finale. I actually think something like that might be in store for the midseason finale of S10. A scene of Dean ready to take the final blow, but then something happening, like something within twisting (his heart) and it would set off a wave of memories of TFW together like teh toy soldier set off in Swan Song for Sam. But that part would only come after mid season. The last episode before the winter break I assume will involve us not knowing whether Dean killed him or not.