In the nature of things, readers are often moved to suggest "topics" for my Discword books. This is somewhat depressing, because most people who are not writers fail to understand how writers think, and if it comes to that, so do I.

Regrettably, football is often on the metaphorical wish list. People don't always realise that a plot is only one of the things that a book needs, and one of the other things is a point. I couldn't see the point of writing a novel about football, a game I have never watched and, when at school, avoided like the plague; I was generally last to be picked before the fat kid. (We came into our own one year however, when we did hockey. That meant I had a stick and lots of advice from my father, who had learned how to cheat at hockey in India, and even the fat kid found a new, well-padded role as a goalie.)

But, like many authors, I research serendipitously, and I enjoy reading about the social history of Victorian England. I came across a little anecdote about the man who invented the pneumatic football, without which the modern game could not possibly exist. I was sufficiently intrigued to look a bit further, and into my head came the phrase "two supporters' clubs, alike in villainy". And within half an hour, four major characters were alive in my head and down on the page. The speed at which the rest of the book was created around them was some kind of tribute, I suppose, to a lifetime's writing.

Generally speaking, if you get your characters right, they will in some way "speak for themselves". In Unseen Academicals, the prime example of this is Glenda.

Initially, I had seen Glenda as playing the nurse role in this football-flavoured version of Romeo and Juliet. In a way, of course, she does, flapping around after her young friend, as my father would put it, like an old hen. But the book really began to take shape for me when she began to think outside the little box of her life. I have known many women like her; they mucked around at school, got married and had some kids, and then realised that they had a fully functional brain, often fearsomely so. They usually find a voice then, as well. Glenda does so, to the extent of barging in to Lord Vetinari's office like an angry mum besieging the headmaster after her little boy has had a telling off. I rather like her, ever since I realised that she didn't know the meaning of some of the more worrying words in the cheap romantic novels she bought, and was ashamed at her lack of knowledge.

Mr Nutt was in a way the seed of the book. Ever since I first read Tolkien at the age of 13, I was worried about the orcs. They were totally and irrevocably bad. It was a flat given. No possibility of redemption for an orc, no chance of getting a job somewhere involving fluffy animals or flowers.

This is no reflection on Tolkien. We are all prisoners in the aspic of our time. But now, I think, people have learned not to think that any race or culture is naturally or irredeemably bad. We have seen the world from space and it isn't flat.

I have waited decades to write about Nutt; I can remember the excesses of football hooliganism that began in the 1960s and have only recently been cleaned up. It was a world of scaffolding-pole clubs and Stanley knives slashing railway seats and faces. The orcs, with a scarf or two, would have fitted right in in those days. More recently, an inflatable banana is the worst thing that's brandished; it would appear that the leopard can change his shorts.

And, of course, as this is a Discworld book, it means that the wizards have to find something to squabble about. Mr Dibbler must try a new scam, Lord Vetinari must plot, in his Machiavellian way, towards a better world, and boy must meet girl or at least drift gently towards her.

Over the years I have endeavoured to keep the Discworld series fresh for the long-time fans as well as for the newcomers. I believe that an Unseen Academicals must be among the more accessible. Indeed, it contains so little of what is popularly thought of as fantasy that in some places it comes close to that strange creature known as magical realism. Various factors made it somewhat difficult to write, and like every book I have ever written, I wish I could have given it a fortnight of extra time, but the editor's whistle was about to blow, so I had to take the shot.

Next week John Mullan will be looking at readers' responses.