First off, I don't like, nor do I read, fan-fics of any sort. I have my reasons that I will explain. Many of the Harry Potter fans write fan-fics. I have heard it said that J.K. Rowling doesn't mind them. I don't know whether that is true or not; I suspect those that feel they have the right to write them continue to perpetuate this myth so that it makes the writer feel better about their writing.
To me, I don't like the idea of anyone "kidnapping" a character. That is, essentially, what a fan-fic is. It's an unauthorized tale of an alternate reality of a character or an alternate storyline for a character. Just as I would not appreciate a well-meaning person borrowing my child to take them to the zoo or fair without my permission, I think the same must be true for borrowing a person's character for the purposes of doing whatever they want with him/her. It doesn't matter whether my child would have fun at the zoo, or that the action of taking my child wasn't done for any malacious purposes, it is for all intents and purposes, kidnapping. Using someone's characters is stealing an idea, a copyright, and doesn't even take into account what an author knows about his characters that may never be revealed, that is, the character's backstory.
Backstories of characters are there whether the reader is aware of them or not. I know my husband, who writes novels, knows his characters' backstories. He says it's important to know their story even if it never shows up anywhere in the novel. That way his characters stay consistent within the story itself. When they don't, the story seems flawed or unrealistic to the reader, and the reader feels betrayed.
Over the years, I have read many of the novels of Andrew Greeley. Actually, right now, I am reading another of his, but back to the issue at hand, which is backstory. His characters remain true from novel to novel. He has the Ryan and Murphy clans that run throughout most of his novels. From having read his book, All About Women, I suspect, though don't know this as a fact, that this collection was the way he roughs out his characters so that he knows his characters inside and out. That collection of stories is basically about many of his women characters. Each story is complete on its own, and yet each basically focuses on who the person is. It was an interesting read because I had already read enough of his books so I instantly recognized many of the women he had written about (deja vu), yet others were entirely new to me. Since then, I have "met" most of them in subsequent novels.
Is that his method for backstory? Perhaps! Anyhow, how do you do it?
Causes Nancy Smith Supports
Doctors without Borders
American Diabetes Association