I can hold onto initial characterizations pretty tenaciously when I don't see the changes as being organic to the story. I've managed a whole season of "BZZT! Wrong! Dean wouldn't really say that!"

Yes. This. It bothers me, but I just excise it when I write him, so I can kind of keep the Dean in my head and the Dean of season 3 separate. Of course, I also don't believe he doesn't know a lot of the shit they have him not knowing, so it's helpful in other ways, too.