The cover for Terry Pratchett’s final Discworld novel The Shepherd’s Crown shows his teenage witch Tiffany Aching facing forwards, arms outstretched, a hopeful expression on her face. But Paul Kidby, the illustrator who has worked on Pratchett’s Discworld novels for more than 20 years, says this wasn’t always the case.

The cover of The Shepherd’s Crown.

Kidby was in the middle of drawing the jacket when Pratchett died in March, aged 66, nine years after revealing the “embuggerance” of his early-onset Alzheimer’s. “My initial sketch of Tiffany has her looking very sad, because that was what I was working on when Terry passed away,” says Kidby, talking on the phone from his home in Dorset. “I reworked it, though, and made her less sad – more forward looking, towards the future.”

Kidby first discovered Discworld in 1993, when he was given a copy of The Colour of Magic for his 29th birthday, and was immediately taken with Pratchett’s writing. After drawing a range of Pratchett’s characters for fun, he took a bundle of them along to a book signing in a WH Smith’s in Bath, and handed them over to Pratchett.

“I queued for three hours – he was so popular and I’d had no idea about it,” says Kidby. “When I got to Terry, I gave him some photocopies of the drawings I’d done in an envelope, and thought ‘that’s it’. He didn’t need another illustrator; he had Josh Kirby, and he’d made it, he was successful. So I resigned myself to the fact that he wouldn’t get in touch.”

But a few weeks later, Pratchett rang him. “He phoned up and said ‘this is the closest anyone’s got to how I see the characters’. And that was it – the beginning. It’s a fantastic endorsement when an author says that … For me, creatively, to do my interpretation of a writer’s character was pretty much top of the tree.”

The published colour versions and the line drawings adapted for the Discworld Colouring Book, of varieties of swamp dragons by Paul Kidby. Illustration: Paul Kidby/Orion Books

At the time, Kidby was a freelance illustrator, painting popular video game characters like Sonic the Hedgehog for magazine covers. He’d always known he wanted to be an artist – as a teenager, he applied to art colleges and was offered a place in Hounslow, but ended up declining it as it was too far from where he lived. But his parents were friends with a retired art teacher and “I knocked on her door with my drawings and she very kindly looked through what I’d done, and took me under her wing”.

On the side, Kidby started making false teeth during the day to bring in some money. “At the time it was the most creative job I could get, as I was still working with my hands. I was 17,” he says. “Looking back over my career path, I was always trying to do something more creative. False teeth were the beginning.”

Kidby variously printed designs on to roller blinds, designed greetings cards, and worked on packaging for lightbulbs and rice pudding. “I did any commercial work that was going,” he says. “It was varied, and I enjoyed the most creative stuff. In the mid to late 80s, I did some video and film packaging, doing portraits, trying to capture the essence of the film. That was more fun. My goal was to stick to doing art. I was trying to improve my ability.”

Paul Kidby, colouring in his own illustration from the Discworld Colouring Book. Photograph: Orion Books

Pratchett liking his work was a turning point. The pair worked together on books including The Science of Discworld, and The Pratchett Portfolio. When Pratchett’s other illustrator Josh Kirby died in 2001, Pratchett asked Kidby to take on the Discworld covers. He started with Night Watch, producing a parody of Rembrandt’s famous painting. Many covers later, it’s still one of Kidby’s favourites. “It was a parody I was wanting to do – it echoed the way Terry would parody things in popular culture in his writing. So it fit in nicely with Terry.”

The pair worked together closely in the early days, with Kidby dreaming up a look for every inhabitant in the Discworld – from six-inch blue Scottish fairies called the Nac Mac Feegles, to Pratchett’s infamous, kindly Death, who looks like the grim reaper and talks entirely in capitals. “He would say if he didn’t like something. Working creatively with Terry was a fantastic opportunity. It was lovely to have that chance to work so closely with him. Although it didn’t happen often, he would say ‘that’s not how I see a character’ and tell me who he was thinking of when he was writing the character,” says Kidby. “I got it with Ponder Stibbons. My initial drawing had him with longer hair, with rounder glasses – he looked more like John Lennon. And Terry said, ‘Think more Bill Gates,’ and I got it and managed to pin him down.”

Rob Wilkins, Pratchett’s long-time assistant, friend and business manager, has called Kidby the author’s “artist of choice”, adding that “Terry often commented that Paul must have the ability to step right into Discworld, because the accuracy with which he depicts his creations often surpassed his own imagination.” And the creative inspiration often went both ways. “When he was writing The Wee Free Men, he said that ‘that character there [from the painting] is Rob Anybody’. He was looking at the painting when he was writing the book, says Kidby. “For me that was lovely, to feel like I’d influenced his writing.”

The publication of Terry Pratchett’s Discworld Colouring Book on Thursday, which recreates Kidby’s art as line drawings to be coloured in, has given him the opportunity to go back over paintings from the last 20 years. He particularly likes Vimes and Granny Weatherwax: “The complicated ones, the ones who are more of a challenge, who you have to push yourself to pin down.”

Tiffany Aching of The Shepherd’s Crown, though, holds a special place in his heart. “One of the reasons why doing the Tiffany Aching books are very dear to me is because they’re set on the chalk downs in Discworld,” says Kidby, who himself lives near the chalk downs in Dorset. “That landscape, which inspired Terry, also inspired me. And she was fun, because in The Wee Free Men she’s eight, and you follow her until she’s 18 – it’s great to draw a character growing up.”

It was, he says, “very poignant” coming up with his image of Tiffany for the final Discworld cover. “I was working on it when Terry passed away and a lot of that went into the painting”. With Night Watch, it remains his favourite out of all the jackets he’s done.

Although he sculpts, and illustrated a book, The Charmed Realm, with his wife Vanessa Kidby a few years ago, Discworld still takes up the majority of his working life, even with Pratchett gone. “There’s a lot to do for Discworld, which is lovely,” he says. “You feel a responsibility. It’s huge, and people love Terry’s writing so much. Hopefully, people will like this colouring book – it seemed to make sense. And it would be a shame not to have a Discworld colouring book.”

Even after two decades, illustrating Discworld is still demanding. “And I really like a challenge,” he says. “What I like most is the diversity of Terry’s writing; it gives me the opportunity to do so many different things. And to make the Grim Reaper look not too scary – that is a challenge.”