I think I fall into the same category as Luna does. For me the most important aspect of roleplaying is staying in character. I think myself into the position of the character I'm playing as, considering their background and personality and then I'll figure out how they would react. While this is a really fun process, it can be a really challenging part if you don't fully understand a character's motives and/or background.
Another problem I run into with this, is fitting characters into a RP. Sometimes you know the general outline of the RP, but it is hard for you to have your character react in the way they are 'supposed to' react, to benefit the Storyline, without them acting out of character. A great example I have for this is playing Frieza in the Dissidia Multiverse RP. While he is supposed to join the other members of Team Chaos and fight for Chaos himself, his personality doesn't really allow him to join forces with others, unless he can benefit directly from it. One option of course is modifying the character, adding Storyline Specific Information that allows him to fit into the RP, or create artificial motivations and distractions to Events, that allow him to stay in character.
All in all I think that Canon Characters are a lot harder to play than self created ones. With Otakus you have created every aspect of their Personality and Background, and can therefore imagine how they would react in each given situation. With Canon Characters, it is much more of a learning process, because you didn't create them yourself, and you therefore have to learn what their motives and personality are.