Have you ever read fanfiction? Fanfics are stories written by fans about a book, film, etc.
They are no comments, but stories that tell you more about the characters that you like very much.
You could write about the adventures of the characters after or before the book, for example.
Fanfics have nothing to do with the real authors of the books, they come from the imagination of the fans.
Interested? Then go to:
There, you can find many fanfics in foreign languages and in German, too. They are about books, films, cartoons, etc.