Generally speaking, I already know what the backstory is. In many cases parts of it are already written. Also, I'm generally far ahead of the publication schedule -- a book or two or three even! So often things are already written long before the book is out for anyone to write fanfic. For example, in Lost Things we never find out what happened to Mitch in the war or what goes on with him. But that's already written in Steel Blues and Silver Bullet.

As to what if I like it -- generally when I like it, it's taking the characters along the same lines I would have. For example, this story of lferion's isn't a scene that's covered in the books, but it could be. It's such a well done missing scene that it could be. And there's a scene that's similar in tone but not in specifics in Steel Blues.

And then there are things that strike out in a completely different direction than I intended. If, for example, someone writes a story about how Mitch was married and she was tragically killed. There's nothing in Lost Things that precludes it. But that's not the direction that we took the character in Steel Blues and Silver Bullet. So it remains an interesting digression, a might have been.

You are very welcome! :)