I think, for me anyway, characterisation of RPF characters is largely influenced by their characterisation in other fics. If I want to try writing a new character/pairing I haven't attempted before, I'll generally go on a fic binge for that character/pairing to get a feel for how other people write them. I do the whole watching interviews thing too (when I started writing Mike Carden/Kevin Jonas, I watched like three episodes of JONAS in one go
) but they're mostly for pieces of information about the character than for characterisation. Mostly I try and write them similar to the way other people write them, but they're never exactly the same because my characters grow into themselves as I write them, and sometimes they change and develop without me actively trying to make them.
Also, though, I think the characterisation of a character varies greatly depending on the plot of the story they're in. I write Brendon Urie differently when he's in love with Ryan Ross than when he's in love with Spencer Smith, for example (though I haven't got much experience with the former) and Mike varies from being kind of an asshole to a really sweet guy depending on the story. Kevin is generally his adorkable self, though, no matter what the story is. I think that's something that should never ever change.