Terry Pratchett: 'If I'd known what a progressive brain disease could do for your PR profile I may have had one earlier'

This is just perfect. Here is Terry Pratchett, white-bearded and diminutive behind a towering double-row of six computer screens, as their greenish glow plays on his face. He looks like the Wizard of Oz.

Nothing in this room dispels the fancy. Giant hourglasses, a huge brass lectern shaped like a soaring eagle, a Gothic fireplace, a fat half-burnt candle in an iron wall-mount, a pointy hat atop a messy ziggurat of papers, a model of a dragon – all sorts of props and knick-knacks litter the place, proclaiming this solidly refurbished country cottage as mission control of a fantasy world.

Appearances, of course, are not deceptive. For years now, Pratchett has sat here typing, adding all-too-human flesh to the bones of his great literary creation, the flat, out-of-time, alternative earth he calls the Discworld. Even now, he doesn't quite want to leave Discworld, and prefers to draw me into it instead. He asks me how many pies one might be expected to make from the rendered body of a large man. I assure him that I wouldn't know, and he goes on, with some relish, to make a convincing case for about a dozen.

He also makes a request of his assistant, Rob, the presentable young man who hovers unobtrusively and protectively. Could Rob just make a note of a metaphor, please? Could Rob just record that a man's sagging adam's apple had the appearance of a chicken's giblets? And one can't help thinking, a little pruriently: "Is that a sign?" Pratchett may have sold something like 60 million books, in something like 35 languages. But for a year now, he's been just as famous simply for being diagnosed with early onset Alzheimer's, aged just 59. That's not what I want to talk to him about though, not really. Actually, I think it's a bit of shame that the diagnosis of a progressive brain disease does more for a man's public profile than the amassing of a distinctive and valuable body of work does. So I refrain from asking about the giblets, and concentrate instead, as we settle ourselves in the adjoining room, on marvelling at all the exotic lumber.

This room is full of stuff too, most of it instantly recognisable as having its provenance in the Discworld. One contraption is really weird, though. Some sort of electronic octopus, bristling with electrodes, is clamped to the back of a leather armchair. This would never appear in the Discworld, because the Discworld runs on magic, not electricity. What in Betty's name is it?

It isn't, explains Pratchett, with some amusement, a prop. It's a slightly outlandish, but nevertheless possibly beneficial theraputic tool, delivering little shocks of electronic stimulus to the brain. "You think, maybe it will work and maybe it won't. What's the harm in trying?" he offers briskly. "Look ... you're a lifetime science-fiction fan ... You've got to have something like that ... It's all there. Not enough flashing lights ... but it's remarkably restful. A friend cast my head so that they could make it so that it fitted me."

Still, I now have the opportunity briefly and politely to enquire as to Pratchett's health, and get it out of the way. He's says he feels fine. But there's a lot more to it than that. I'd been worried that Pratchett's disorder might silently dominate our interview, like the proverbial elephant in the room. But the elephant's gone rogue already, and is crashing about our encounter as if such a meeting could have no other possible purpose. Pratchett talks beautifully, as beautifully as he writes, and once he starts, there's no stopping him. He's started.

"I have posterior cortical atrophy – PCA – which is an odd thing that affects visual acuity. No one would know unless I told them. About now, I might be going to the doctor's and saying I think there may be something wrong, if it wasn't for the fact that it hit my typing, spelling and handwriting. We're just coming up to the anniversary of that.

"I don't know if you can arrest development. Aricept [the first drug to be licensed in the UK specifically for the treatment of Alzheimer's] helps, for example, walking helps, and a busy and active life appears to help. But you don't actually get better from it. One of the things that I believe does happen is that if you have, as it were, the physical resources, consciously or unconsciously you come up with workarounds."

This is difficult. On the one hand, it's really wonderful that Pratchett is so willing to talk frankly about an illness that is so greatly feared and so hugely stigmatised. On the other, this is all in the public domain already, and there are so many other things to discuss. I assure him that he seems just fine to me, so maybe the precautions are working. It's a puny little interjection, and Pratchett is nothing if he is not a precise thinker. So this is an open invitation for him to elaborate.

"Well, there are two schools of thought. My PA thought it was that I'd had all the metal fillings taken out of my mouth. It could be the orange juice every day. Both my daughter and my wife thought they had detected improvement. My feeling is that, yes, with Aricept there could be some actual improvement. But it's also the fact that I was aware that there were problems, and so put more mental time into and effort into doing things right. I was having difficulty with seat belts last winter, while I was wearing heavy clothes ... Stab, stab, stab, stab. In spring ..." He makes a noise like a cartoon plane coming in to cartoon land, and makes a swoop with his hand ... "mmmmmyoing ... click! Like all these things, it's anecdotal. Would that I could remain like this for ever."

Oh, lordy. What do you say in response to such a wistful, straightforward, impossible wish? I'm paralysed with useless sympathy, and it's written all over my face.

"This is turning into the classic Alzheimer's interview," Pratchett continues, crisply. "I've done that 60 or 70 times. Why am I doing it now, after the better part of a year since I came out and said it? It's not like it's news any more. ......... f Having occasionally done other things with my life other than contract an unusual form of an unusual disease ... I may as well leave the disease in this chair, and you can interview it, and I can get on with some writing. If I'd known what a progressive brain disease could do for your PR profile I may have had one earlier."

I say that I really couldn't agree more, and absolutely beg Pratchett to stop saying interesting and provocative things about his medical situation. He pays no heed.

"Oh, I was being cynical. It never occurred to me not to speak about my diagnosis. And I thought it could do some good. I didn't think there would be the furore that there was. I've been engaged in a lengthy documentary with the BBC, which has been quite exhausting, and I've done lots of things for Alzheimer's charities and so forth. I hope we're now coming to the point where, OK, we've made quite a lot of splashes and now things might just settle down a bit as we head towards Christmas. I think I've got to go and deliver a petition to 10 Downing Street or something shortly, there's a couple of other things we need to tie up ... it wasn't shall we say, how I saw my life ... but I can't conceive of an alternative universe where I didn't tell anybody."

It's the encroachment on the time that he usually spends conceiving and refining alternative universes, that bothers Pratchett, who reckons that the time he has spent publicising Alzheimer's has cost him "half a book". It would cost most writers more. Pratchett published his first book when he was 20 and has sometimes published three books in one year. He worked first as a local newspaper journalist and then as a press officer for the Central Electricity Generating Board before commerical success freed him to write full-time in the mid-1980s.

And Pratchett is not knocking out light entertainment – even though his books are both entertaining and accessible. He is a serious and driven artist as well as a populist storyteller, engaged, intelligent and intensely creative. Also, he is insightful enough to be able to understand and vocalise his creative process, which is in itself a fairly unusual thing for a writer to do.

"On the latest Discworld book I was struggling mainly because of time – doing it in bits – and a couple of nights ago I hit the point which I call Delta Star. It's a physics term and I can't remember what it relates to. But suddenly, there's a point ... You've got your characters working and you know how a book's supposed to go. You think there are things that you don't quite know because you should always be open to what is scientifically known as emergent behaviour.

"It usually happens because a character says one sentence to another character and somehow, yes, it is around that quote that this whole book is now starting to spin, not because that quote is particularly memorable but it was exactly the right word at the right time. I now know a lot more about this character, and now this character knows a lot more about himself."

or all those uninitiated into the Discworld, this might seem odd. The props around Pratchett's office, his self-classification as a fantasy writer, these conspire to suggest that Pratchett's books are escapist trivia, sub-Tolkien nonsense peopled by trolls and elves, suitable only for children and childish adults. (Pratchett does write many books for children, and his adult novels are known to be attractive to "reluctant young readers".)

In attempting to get across that Pratchett's oeuvre is not simple escapism, critics often refer to it as "satire" or "parody". Certainly, there are satirical elements in the work, like Pratchett's prescient invention of a Discworld local-government tourism leaflet, many years ago, entitled: "Soe you're A Barbariean Invader?" But that's not all that's going on, as Pratchett agrees.

"Satire of what? They think that satire is going on, but not of anything. Authors I particularly admire are Mark Twain and Jerome K Jerome who wrote in a certain tone of voice which was humane and understanding of humanity, but always ready to annotate its little foibles. I think I'd lay my cards down on that, and say that it's that that I'm trying to do."

Twain and Jerome wrote naturalistically, about our world, and Pratchett does not. Nevertheless, like them, the great strength of his work rests on its accurate, sympathetic, humorous portrayal of human character – and sometimes lack of it. His vast, eclectic readership proclaims that this approach works. But still, it all begs the question: why genre fiction, why fantasy, at all?

"When you were a kid," he says, "you'd have a paint box and you'd take it to school. But there was always the rich kid, and he'd got the paint box with the silver and the gold and possibly the turquoise as well. Instead of doing the best you can with the colours you'd got, you really wish you had the colours he'd got. Fantasy gives you the silver and the gold and the turquoise."

Yet in many of Pratchett's Discworld novels, the silver, the gold and the turquoise are incidental to the human story. When pressed on why he has never tackled naturalism, this quietly confident man says quite an odd thing. He says that he would have written one Discworld novel, Monstrous Regiment, in our world, if "I had had the nerve".

Monstrous Regiment, as is usual with Pratchett's novels, is based on meticulous historical research, this time into the many stories involving women who for various reasons have disguised themselves as men and gone to war. Again, as is usual with Pratchett's novels, it is richly comic, and takes this concept to a reductio ad absurdum.

"It was great, great fun to write and I've had lots of letters about it. I could have set it in the Napoleonic Wars ... I'd have had to invent a few things, but it didn't really need to be Discworld. I get a certain amount of extra fun with it in Discworld and I could take liberties in Discworld that I couldn't take in a real historical novel. Actually some of the things I found out were too surprising even for Discworld ..."

In an odd sense, most of Pratchett's books are historical novels, as well as being fantasy novels and comic novels, because he draws so much on the past to feed his imagination. Crucially, though, their rounded humanism emerges because Pratchett is freed to treat religious myth and pagan folklore as equally influential and significant elements in the human story. He writes about how humans actually behave, but his observations also incorporate the "real" fantasies that humans have always made up for themselves and each other. This habit developed early and naturally in Pratchett's life as a reader and a writer.

"When I was a kid I read the science-fiction shelves, and I read the fantasy shelves. There wasn't too much of either in my library, because neither genre had developed at that time as much as it has today. So you go on to the folklore because that's a bit like fantasy. You go on to the Greek and Roman myths and legends because that's a bit like fantasy. You go on to the dinosaurs because that's a bit like science fiction. You go on to ancient history because that's a bit like fantasy.

"Next thing you know you're reading about the Silk Road and the Silk Road, because it's a road, leads you on to other stuff. So you start reading about the Chinese empires. One book leads on to another and little bits of knowledge hook together like some kind of DNA strand and you're getting some kind of an education. Sometimes it makes you seem like a twit but nevertheless all this stuff is happening.

"I get an awful lot of letters asking: 'How can I grow up to be a famous fantasy writer like what you are?' And you say: 'Stop reading fantasy right now. Read history, but not queens and kings history – social history. The history of the privy, the history of cod, the history of the frozen water trade in North America ...' Read stuff from another world, and the closest other world we've got is the past. The past was hideously different from how it is now."

But Pratchett's originality relies not only on retelling the past, but also on re-examining the impossible. One of the long-running characters in Discworld is a Werewolf who works in the City Watch, which is the equal-opportunities police force of his sprawling fictional metropolis, Ankh-Morpork. Pratchett spent much time discussing with an eminent biologist how werewolves work. "You don't have to do a scientific treatise here. Because we know there is no such thing as a werewolf. But culturally, through novels and the movies, we have a vague idea what werewolves are and how they work.

"Can we invent a physiology, a history for werewolves that fits in with all those things that popular culture has said werewolves are? Do werewolves have kids? What are they born as? What happens if a werewolf marries a human? All these things are really, really interesting because you can talk about other things in the guise of fantasy. You don't actually necessarily know you're doing it, but you're usually dealing with something actually germane to us in the here and now. But you are doing it through sock puppets – carefully written sock puppets.

"And indeed, actually, thinking seriously, that's what Discworld is. Discworld is taking something that you know is ridiculous and treating it as if it is serious, to see if something interesting happens when you do so."

ratchett, of course, does think about the something ridiculous that is one day finding himself unable to conduct this process anymore. He says that he is fairly stoical about the prospect, and is concentrating on taking things one day at a time. The 37th Discworld book, Unseen Academicals, has achieved its Delta Star, and Pratchett is also very much enjoying his long-held ambition, of seeing the Discworld being brought to the screen, by a small British company called Mob Films. For a long time Pratchett nursed ambitions that the Discworld would eventually get the Hollywood treatment, but after many frustrating dealings, he opted instead to work with a company that offered an opportunity for Pratchett to be fully involved. So far, they have adapted The Colour of Magic, The Light Fantastic (the first and second Discworld novels) and The Hogfather (the 20th Discworld novel) together. Pratchett is delighted with the results: "They're done with love and care, not enough money to bugger it up and probably just enough to get it into the area of being right."

Intimate, small-scale creativity has had a number of happy results. "I could go on-set not only to make suggestions but also to be listened to. We even ... at one point there was a big crowd scene and someone said: 'Would the fans like to be in the crowd scene?'

"There's a huge history about fans of all sorts having taken part in movies, most of it bad, but we announced it, there was a kind of lottery thing, and they were absolutely as good as gold ... When another crowd scene was called for, they said, 'Can we have the fans back because they're such good extras?' Because they know what it's all about. You don't have to say to them, 'This is the Mended Drum [Ankh-Morpork's liveliest hostelry].' They say: 'This is the Mended Drum! I'm drinking in the Mended Drum! Where do you want me to sit?' They were volunteering for everything. They were absolutely magnificent."

Pratchett has a lot of dealing with his fans, not least because he hosts a Discworld convention every two years. Again, the habit started in his own childhood: "I attended my first science-fiction convention in 1963." One of the poignant details of his condition is that he is no longer able to write personal dedications when he is signing books.

When he talks about the readers that he meets with, Pratchett talks with warmth and pride. He was especially delighted by a session he recently introduced to his conventions, aimed at young readers of the Discworld. He is often asked if he really believes that his novels are suitable for young readers, and even sometimes whether the books he writes especially for them have entirely suitable themes.

"My advice is this. For Christ's sake, don't write a book that is suitable for a kid of 12 years old, because the kids who read who are 12 years old are reading books for adults. I read all of the James Bond books when I was about 11, which was approximately the right time to read James Bond books. So you work out this kind of little equation in your head and you think, yeah, like Nation – the one that's just come out – that's a book for kids. And people will say: 'Well it covers very adult subjects ...' Yeah, that's why it's a book for kids. Because you want kids to grow up to be adults, not just bigger kids." And that too, is something special about Terry Pratchett and his work. Without pretension or crusading auto-didacticism, he has spent a quarter of a century creating a fantasy world that does its bit to make this world a better place. It is a splendid achievement.

'The Colour of Magic', produced by Mob Films, is now available on DVD and Blu-ray