Author has written 4 stories for Harry Potter.
I'm by no means a professional writer, and I'm not a great reader of fanfic. But my field of research is the history of magic and religion, so I think I have a fairly good understanding of the depth and complexity JK Rowling relies on in the Harry Potter series. Being also an analytical person who likes to understand why and how people behave the way they do, I have formed my own theories as to the likely course of events for some parts of the story which aren't described in detail in the books. My fanfic is simply a way to put those theories into a format and forum where others can enjoy them (hopefully! LOL) and give feedback.
In my opinion, the most persuasive fanfic follows these guiding principles:
Firstly, the facts must be consistent with facts in the canon. Generally speaking, the licence which an original author has to write in contradictory details does not extend to fanfic writers.
Secondly, the characters must be consistent, in conversation and behaviour, with their presentation in canon.
Thirdly, physical descriptions of objects and people in fanfic must be consistent with their descriptions in canon.
Fourthly, any new characters or storylines introduced in a fill-in story should be consistent with canon and yet not so striking that their absence from later canon is inexplicable.
And lastly, the style of writing should be as close to the original as possible, in order to allow the fanfic reader to forget that the story is not canon.
Following those principles requires a lot of reading, cross-checking different references to events in canon, close attention to detail, and careful writing. The basic concept for the story may come to me in a flash, but the writing can be days or months of careful crafting of the tale - depending on how long it is, of course. A 5000-word story based on a single reference in canon is likely to take me only a few days to craft. A 20,000-30,000 word story based on, say, 10 mentions in canon might take two or three months of reading, writing and cross-checking.
Currently in progress: a 9-chapter account of the events from Harry's rescue from the ruins of the Potter house to his being deposited on the doorstep at Privet Drive.
Next: An account of Voldemort and Bellatrix's relationship.