Thanks! Yeah, it's actually coming more easily than I was afraid it would. I guess writing that silly fic first really did help me get to grips with things. That and I've spent way too long thinking about the main character involved for reasons unrelated to the fic anyway, so I probably shouldn't have been anywhere near this worried in the first place.
Regarding what you mentioned in the Mafia thread (since I don't really want to derail the thread by replying there), I actually don't think I want to quite reach the point of never having to consciously think about the actions of a character that isn't mine. I worry that if I started doing that, it'd only be because I've slipped away from their canon self into an ever-so-slightly-different fanon version of them that only exists in my head. And that would be baaad. Like I said, I don't actually mind having to consciously check a character's actions against my understanding of them, because it's fun! Usually it's not even to correct myself - it's just to confirm what I'm writing, like, "Yes, I am confident they would act in this way to this situation, because of X, Y and Z from this part of the canon".
(And you're not being pretentious at all! You are a perfectly competent, visibly improving writer who has every right to give advice to someone if he thinks it'll help them. Besides which you probably have more recent writing experience than me, anyway. =P)