Don't break Joss's toys.

Passarella also draws attention to another vital point to consider - by the end of the book, leave your main characters as you found them at the start. "You are playing with somebody else's toys and when you're done with them, you have to put them back in the toy box in the same condition you found them.

"The writing staff of both shows have the luxury and the flexibility to evolve and mature the characters and their relationships. The authors of the novels lack that same flexibility. Again, the constraint is to maintain past continuity. This is probably the main reason for the 'new' characters that the novelists add to the mix in the original novels. The reader knows that the 'show' characters will emerge, ultimately, unscathed or at least unchanged from how they see them each week on television.

"So the invented characters add the element of the unknown, the wild cards in the deck the tie-in novelist plays with, whether the new characters are protagonists or antagonists. The authors have the opportunity to create sympathetic characters who can be placed in 'legitimate' jeopardy and, of course, to create hideous new villains. These two areas give the writer the most room to flex his or her creative muscles."

One of the plusses of working with established characters is that there's a little bit of creative shorthand involved. "I don't have to invent the world, just learn its rules," says Passarella. "I don't have to create the main characters, just learn their behaviours. As a media tie-in novelist, my goal is to create an exciting new story in the pre-existing world I'm given while remaining true to the characters and relationships.

"Although the disciplines vary between writing original novels and media tie-ins, for a first-time novelist, the latter may be the slightly easier path to take, for a couple of reasons. Aside from the relatively shorter length, some of the world-building is already finished for you. You're working with a net. Still, you will gain experience in creating your own characters, developing plots, outlining and completing book manuscripts.