To me, exactly this--how to deseminate character info--is the number one most difficult thing about writing. In short stories, I think the answer is easier: you give exactly as much as the reader needs to know. But the reason I love both reading novels and writing them better than short stories is it's not about a point you're getting to, it's about this whole world you're exploring. Yeah, the details should still be important to the over all story, but they're not essential. So, how much do you include, and how do you include it?

One thing that's easier about writing fanfic is you really can just dump a reader into the world with very little explanation--they won't be confused by names and places and backgrounds because they know all that already. In an original novel, it's very difficult to do that without it being annoying and confusing (one thing I often dislike about much sci fi and fantasy). But the more difficult thing about writing fanfic is that once you do want to go into exploring the internal workings of the characters--how much is necessary to explain, since the reader really already knows the characters? With original fic that's not a problem because you know exactly how much the reader is bringing to the table (i.e., nothing, in most cases). Still, like I said, I think the most difficult thing ever is spooling out that information in a way that is interesting and useful, instead of either dumping too much or withholding too much.