How many people have read a fantasy series and been completely swept away by the characters and setting? It's part of  what makes fantasy such a popular genre. You get to completely create a new world filled with whatever your imagination can dream up.

When an author creates a world that you love and would do anything to be part of, it often spurs other writers to write fiction in that universe. It doesn't just happen with books, but television as well. Fan fiction sites dedicated to fantasy worlds, such as Middle Earth and Riverworld, are all over the Internet.

While technically they could be considered copyright violations, authors and book companies don't care as long as no one is profiting from the stories. Many a writer has started his career writing fan fiction for his favorite book series or television show and discovered he had a knack for crafting the written word. 

Sometimes it move beyond just fan fiction and turn into a real job. Who do you think writes all those Buffy the Vampire Slayer and other books? They are authors who love the story and wanted to create their own to keep the characters and worlds alive.

I know a few authors that not only created some of these novels, but actually collaborated with some of the actors. Authors love fan fiction for the most part. It's flattering when someone tries to emulate your writing style, so don't be afraid to write your own fan fiction. Who knows where it might lead?