David Artsmith  -  When you choose to take a piece of literature and expand upon it, either through roleplaying, or through some form of fan fiction, there are a number of choices that you have to make. The very first thing that you have to consider, is whether you want to stick to the original tale as told by Rowling, or if you want to branch off into speculative fictional ideas? If you do want to follow the original storyline, you can choose to either act out something which has already happened, copying a chapter or several chapters from the book, or you can choose to work around the story, telling the tale of people whose actions weren't explicitly described in the book.

On the other hand you can always choose to create a completely new derivative. You can make Harry Potter turn evil, or can have Hermione get a crush on Snape, or any of an infinite number of changes that you can make, to explore the "what ifs" of the story.

Another choice you have to make is time. A series such as Harry Potter stretches out over 8 years, and you can set your own derivative adventures during any of those years, or even before or after the events in the book.

Finally, you have to make choices about characters. You have the whole universe of Harry Potter characters open to you, but you have to understand their relationships, their connections, and how they react and interact with one another. This will all also be determined to some extent by the time period that your story is taking place in.

One of the things that can make a decent Harry Potter roleplaying session so difficult is that the book series is quite long and detailed, with literally hundreds of characters popping up at various points throughout. On the one hand this gives you a lot of room for creativity. On the other side of things, this increases the potential for you to make errors, or to leave out important information.