"I feel like this Leviathan Cas is what happens when you mix Misha and tequila." Well, it's actually what happens when you mix Misha with a bad case of the stomach flu cause he was really sick when he shot this scene.
"Cas is well 'hmm' gone." In 8 seasons i have never been as creeped out as I was in this moment. It was still Misha acting, you could see that, but in the space of seconds it had gone from Castiel to this taunting evil thing that was speaking through him.