I know a lot about Stargate: SG-1. It's my favorite television show ever. Even so, it took me a long time to feel like I was capable of writing fanfiction for SG-1. I didn't feel like I had enough of the characters' life stories or understood enough about the Goa'uld or 'gate technology. If you don't watch Stargate and have no idea what I'm referring to, then substitute details from your own favorite franchise.
Can you write in the characters' voices and hear them the way they sound on the screen?
Can you remember details of their lives or the things they've said about "how stuff works?"
Well, neither can I--at least not perfectly. Neither can a TV's show's writers or the script writers for sequels. (Look at the Highlander film franchise and its huge continuity problems for an a movie-related example.)
I first got involved in the Stargate fandom around 2005. I met people who were way more knowledgeable about the show than I was, and when I started thinking about writing fanfiction, I asked a lot of questions. I was probably annoying. That's okay, because it made my stories better.
There were times, though, when I found that nobody knew what the answer was--or there were so many different answers circulating the fandom that I might as well just pick the one I liked best or make up my own and run with it.
You'll eventually get flack for that. Somebody will come along and leave a review to tell you what you got wrong. It may even be someone you know--someone you normally go to when you have questions. That's one of the things that makes transformative writing different from most other kinds. The audience gets to interact with the source material and with the new stories that are being created around it.
So am I saying that fanfic authors should just make up whatever they feel like? Not at all, but here's what I've learned from all the questions I asked:
Sometimes the answers you need just aren't available in canon. If nobody seems to know for sure, pick something, make up as much detail as you need to, and go with it. Just make sure that what you decide on is plausible, and stick with it. Also, don't obsess too much about the things you don't know. Fill in as much detail as you need to and leave the rest alone. In most cases, nobody else knows either, and fanfiction is supposed to be fun.