But the funny thing about that is we (as readers/viewers) sometimes miss out on information that might have been interesting. The author didn’t think it was, but fans? Most fans will soak up content like a sponge (see: LotR extended editions, cutscenes, etc). And so we’re likely to ask ridiculous questions like “What is laundry day like at Avengers Tower?” - not because it’s important to the narrative, but because we’re curious.
Not to mention: every narrator is an unreliable narrator. Especially the ones who seem the most straightforward. Which means there are a wealth of stories not being told hiding right behind the story that is.
Which, I think, gives an inkling of the primary difference between original fic and fanfic: original fic is declarative, saying “here is the story, these are the important events and characters and aspects of the world,” while fanfic is exploratory (even when it’s got a cracking good plot).
Fanfic exists in the interstices, in the ellipses and the enjambment. Fanfiction exists in the moment before the wave function collapses. A transformative work doesn’t actually transform the original media it is based off of (because the original medium exists in a fixed state and cannot be literally changed by fans unless the canon creators allow it to be so) so much as take the essential structure of the narrative and the characters and twist it, turn it, rotate and reflect it until we’ve built a fractal around it."