Maybe the key is to bring the lurkers out to play. This might help.
Fanfiction Writing 101
by Distracted, who doesn't do this for a living. Because of that, this advice is just that... advice, not rules. And it's worth whatever you think it's worth, since it's free.
Step One: Watch the show. A lot. So much that the characters in it become real people in your mind, people with their own personalities, mannerisms, motivations, and histories. Watch the entire show and learn the timeline of the episodes (or use a handy dandy reference guide) so you can place your story in context.
Step Two: Decide what story you want to tell. Most fanfics are born because the writer wants to see a continuation of a scene that interests them or wants to fix something that they think the writers got wrong. If something bugs you, fix it. If nothing bugs you, then pick a classic plot trope and run with it.
Step Three: Choose your point of view character. Telling your story from a single POV isn't essential, but it makes for a more gripping narrative. Readers instinctively want to identify with the person telling the story. If you don't give them someone to identify with, it creates distance, which may decrease the emotional impact of your tale.
Step Four: Pretend that you are your point of view character. Let your story play out in your head. Write what you "see".
