Fan fiction (aka fanfic) is fiction which is inspired by an existing piece of work; usually a book or film. Songs and poems are possible sources too. A songfic is a story in which the lyrics of a song are intertwined with the text.

There are many places on the Internet where you can find fanfic. Your first stop should be fanfiction.net, where there is a vast collection of material from many fandoms. It's truly immense. After ff.net, look for sites devoted to the particular fandom that you are interested in. You will quite likely find stories on those sites, or pointers to fanfic sites dedicated to that fandom.

If a work such as, to take a couple of examples, Star Trek The Original Series or The Lord of the Rings has caught your imagination and you'd like to read more about the characters and situations that feature in it, you're a bit stuck. I mean; Captain Kirk has hung up his phaser and JRR Tolkien has passed over the Sundering Seas to Numenor. There's never going to be any more stuff written or filmed about any of the people in those stories. Not by the original authors, anyway.

But it might just be that someone else has written about them and those stories might slake your thirst for more sequels or prequels. It's got to be worth a look. How did Jim Kirk get into Starfleet Academy? What happened when Legolas and Gimli visited the Shire? Wouldn't you like to know? Wouldn't you just? A shot of fanfic may be exactly what you need.

I'd better insert a warning here. There's a rule of life that I know as Sturgeon's Law, after the Science Fiction writer Theodore Sturgeon. It says: 90% of everything is crud. There, I've said it. Let the reader beware and remember that he is getting exactly what he has paid for.

Why indeed? Why does anybody want to write anything? That's a question that's rather too big for a little article like this. The answer is in your head, not mine.

But let's make one thing very clear from the outset. You're never going to sell your fanfic. In fact, you're infringing the original author's copyright when you write it. Those are his characters you're playing with and his worlds you're exploiting. You're taking advantage of his hard work.

An author is perfectly entitled to demand that you take down any fanfic that's based on his material and that you haven't licensed from him. He can send you lawyer's letters if you don't comply. You could end up being sued.

In practice it's not as bad as that. Fanfiction.net has a short list of authors whose work you may not use. Most writers, songwriters and film-makers have got enough sense to realise that the writing of fanfic is an integral part of an active fandom. Who is going to be so foolish as to try to shut down a fan activity? Lawyers and money men who are afraid that if they don't take action to protect their IPR they will lose it, as Bayer lost the exclusive right to call their pain relief medicine Aspirin, is the answer. So long as you're not out to make money off the back of their work, most sensible authors won't try to prevent you from writing or posting fanfic based on their books, films and songs.

Good grief! If it were me I'd be dead chuffed…

So, to get back to the question. Why? Here are a few possible reasons:

• You hated the way the story ended. Let's rewrite it the way it should have gone.
• You really liked a character. You'd like to find out more about him or her and writing their story yourself is a great way to do it.
• You really liked a character and you'd love to get off with him or her. I'll say more about Mary Sue in a minute.
• You really liked a pair of characters and you wished they would get off with one another.
• The author left great big holes in the plot. If he won't explain them, you will.
• In the case of SF or fantasy, you're fascinated by the world the author created and you'd like to write some stories of your own set in that world, using some of your own characters.
• You'd like to mix and match characters from more than one fandom. This is called "crossover".

Try that lot for starters. Now - time to get personal. I've written (at the last count) over 300,000 words of fic inspired by Philip Pullman's trilogy His Dark Materials (Northern Lights/The Golden Compass, The Subtle Knife, The Amber Spyglass). That's one heck of a lot of words, spread over nearly three years (at the time of writing this article). Why have I spent all that time writing stories that I'll never be able to sell?

It's terribly simple. I read the first two books and thought they were some of the best stuff I'd read for ages. Great world-building (I read a lot of Science Fiction), great ideas, great characters, great story, great imagination. Then I read The Amber Spyglass and I was totally blown away. It's hard to underestimate the effect of this book on the susceptible reader (some readers are left completely cold by it). It finishes with an emotional one-two punch that left me, and other readers I've spoken to, reeling.

I shall be forever grateful to Philip Pullman. His books gave me the inspiration and impetus to get back into writing stories after an hiatus of nearly 30 years. I wrote The History Tutor in a single day less than a month after finishing The Amber Spyglass. The Reliquary followed a week or so later and gave me one of my most enduring characters in Arthur Shire. After that it was hard to stop. I got caught up in my own stories. The characters demanded to be heard, and who was I to deny them that?

How do I write it?

Firstly, and most importantly, the usual rules of writing still apply. (By the way, I don't know what those rules are). Your stories had better be interesting and your characters believable. You'd better not litter your text with typos and grammatical errors and it had better be decently laid out so it's nice and easy to read. Just because it's "only a fic" doesn't mean you can be sloppy in your writing of it.

In one way at least your task is simpler than it would be if you were writing an original story. Your readers are already familiar with the most significant characters and the world they inhabit. You won't have to do a great deal of background-laying and character building.

But - and this is an important but - you must be absolutely familiar with your source material. You will deserve to be torn apart if you write about a young wizard called Henry Potter or put Doctor Spock on the bridge of the USS Enterprise. Stick to the canon, unless you know exactly what you're trying to do.

OK - now, what kind of fic are you going to write? You may be a "shipper"; a writer who likes to set up relationships between original characters. So - what if Martin Crane and Daphne Moon had taken a fancy to one another and Niles Crane got to hear about it? If it's a homosexual relationship you have in mind it's usually known as "slash". This has nothing to do with Guns and Roses but refers to the way such relationships are described - e.g. Legolas/Boromir. Such a relationship could be anywhere on the scale from wistful thoughts to full-on… you know.

Legolas. Hmmm, let's bring on Mary Sue. Mary Sue is a lightly-disguised copy of the author. He or she appears in the story and before you know it they're giving the author's favourite character all they've got in a full-blown orgy of wish-fulfilment. Wonderful fun for the author - rather a bore for the reader. Keep her out of your stories, why don't you?

To my mind, the primary decision you have to make is whether you are going to work with the existing characters or introduce new ones of your own. That's your choice. If you're itching to write about the further adventures of Lyra Silvertongue then go ahead. Do remember to stay true to her character, though, and don't put her in an alien setting unless that's the point of your story.

You might choose to write a fic that features only your own characters. This is risky, in that there's a pretty good chance that nobody will want to read it. You may as well write an original story and keep the copyright.

Personally, I've found that I get the best results by letting my own characters interact with canonical ones (back in your corner, Mary Sue!) or developing minor characters from the canon (step forward Adèle Starminster and Gracious Wings). Your mileage may vary.

Try to keep the style and rating of your story somewhere near that of the original. To write a Sherlock Holmes story in Mallspeak or describe an XXX-rated encounter between Ron and Hermione is to go outside the concept of the original story and its intended readership. It may work or it may not, but you're pushing the limits. Yes - that's what limits are for, but be aware of the effect you're trying to create.

It's up to you. You may have decided already that the game isn't worth the candle; that writing for no profit isn't for you. Fine. Alternatively, you may find that with the pressure of writing something that needs to be publishable removed from your shoulders your imagination takes flight. If your writing has become a chore, if you need a break, if you'd like to write simply for pleasure or to try out a new mode (future-tense, second-person, unreliable narrator!) in a zero-risk environment, then fan fiction may be just the thing for you.