I think it's safe to assume that the vessel only has to give its consent in the beginning, and then consent is no longer necessary. I mean, when Castiel popped out of Jimmy, I could see that Jimmy had been pretty sick of being a vessel for a while before he was finally himself again.
Then again, there's also every possibility that the vessel's consciousness is put into a kind of deep hibernation while the angel is in the drivers seat. Jimmy said that he didn't remember a whole lot, right? Maybe he only remembers "bits and pieces" because he was in a consciousness coma, and only had the sense of being "chained to a comet" when he came out of his coma for a moment.
By the by, I was happily impressed with the episode last night. Though I think the children shouldn't've been directed to attempt the "evil smirk" (since, in my experience, children just can't fully pull it off) and I think the Batman voices used by fake!Sam&Dean got annoying after a while, otherwise it was actually pretty brilliant.
OH MY GOD, Chuck! Chuck!!! I heart him so very very much! And Becky, she's hilarious! I have to say, I reacted in exactly the same way as Becky when Chuck swooped in, swung that iron weapon like a freakin' warrior, and then went all bad-ass and commanding. Holy crap, SWOOOOOOON. And dude, Chuck/Becky... that pairing made me stupidly giggly. :lol:
There were a TON of funny moments, too, and they weren't all as uncomfortable as I thought they'd be! Hooray!
One more thing - I like that fake!Sam&Dean taught real!Dean a life lesson... and Dean actually looked contented and accepting for a short moment in the end, and it made me ridiculously happy. We don't see the boys smile like that anymore, so seeing Dean happy, if only for a moment, was outstanding.