Interesting! I am not completely sure that fanfic doesn't have some of the work done already -- what I have noticed as the primary difference between my fanfic and my original fiction is the different way in which I introduce characters. In fanfiction, I can basically say something to the effect of "and then Person A walked into the room" and know that readers will fill in Person A's personality and story role for themselves without me having to make all the details obvious. In original fiction, I don't have that shared pool of knowledge/assumptions to rely on, so I have to do more of the work upfront. It's like, instead of people seeing the story shadow and saying, "Oh hey, I remember this pattern," I am showing a completely new shadow. Or something, I don't know, I think this analogy got away from me somewhere.

But yes, in both cases you HAVE to know the shape of the world and the characters in order to cast a realistic shadow. And you have to play by the rules. That's another way fanfiction might be trickier than original fiction, because in original work, you get to make up the rules and therefore ensure that they're congenial to your own worldview and stuff. In fanfiction, you may be forced to play by rules you utterly disagree with... and working within those constraints, and figuring out how to express disagreement while still casting a story shadow that recognizably overlaps the canon story, is not a negligible challenge!

(Sorry if this makes no sense. I am ill and currently dosed on NyQuil and Benadryl at the same time, which I have been told produces a fairly convincing facsimile of being stoned.)