Gruffalo and Tiddler illustrator on the 'imperfections' of his work

Writer Julia Donaldson and illustrator Axel Scheffler are among the most successful duos in children's literature.

Their creation The Gruffalo - about a mouse who invents a fearsome friend to scare off predators, only to learn the creature is real - has sold more than 18 million copies and been translated into 60 languages.

Other works like Room on the Broom, Stick Man and Tabby McTat are also loved by children around the world. And their story Tiddler, about a tall tale-telling fish, is the latest to be adapted into a film for the BBC this Christmas.

But what has made his illustrations so popular? Scheffler says he's not sure.

"I'm somebody who's not a perfect draughtsman," he says. "I think there's a certain imperfection in my drawings that might be appealing."

These "imperfections" also stem from Scheffler doing all of his drawings by hand.

"I can just about do my email and that's about it for my computer skills," the German illustrator says.

"The editor still gets the pictures on little scraps of paper. I can't do Photoshop, so if I do a sky separately and want to put it in, the publisher does it for me. But most images are the originals that look exactly as they do in the book."

One way he keeps things interesting for himself is by including details he describes as "a little hint that it's not a perfect world".

Eagle-eyed readers of Tiddler, for example, might notice the occasional piece of man-made waste among the undersea world.

"They're little hints that although our books have to be quite cheerful and have a happy end, there's things wrong with the world," he says.

Scheffler explains he doesn't know what author Donaldson is working on until he is sent the latest story. This is just one part of their creative process, which he describes as "quite unusual".

"The only interaction we have while we're working on a book is via the editor. So that is the story of our collaboration. It's never changed," he says, sitting in the kitchen of his London home in front of a cabinet painted with animals and birds in his signature style.

"We became friends over the years, but we never call each other and say, 'Can you change that?'"

Asked if this might be the secret to their success, he says: "I have no idea. It's just how we did it and it worked really well. It's not a formula that can be followed.

"But yes, I think the illustrator has to have freedom and autonomy."

Scheffler first got into illustration in his teens, when he dabbled in political art that he now dismisses as "embarrassing things about the Vietnam War".

After leaving university and becoming a commercial illustrator, he quickly learnt his style was better suited to children's picture books.

In 1993, he worked with Donaldson for the first time on A Squash and a Squeeze, a book adaptation of a song she had written for the BBC's Playdays series.

Their biggest success came in 1999 with the release of The Gruffalo, which grew from a word-of-mouth success to a worldwide phenomenon.

"The amount of merchandising is a little overwhelming for me," says Scheffler. "I get a big box every six weeks or so. There's an overflowing room downstairs."

For over a decade, the duo turned down nearly every offer related to the character.

Although Scheffler says he's "ambivalent" about the commodification of the character, he adds: "It's nice for children to have a cuddly toy or a Gruffalo on their jumper."

The amount of merchandising, however, is testament to how much people love his illustrations.

Asked whether he would ever change his signature style, he says: "Most illustrators have that feeling, especially if they've drawn this way for 30 years."

But fans shouldn't expect any radical departures: "I could probably say I want to do something, but that would require time to develop something. It's much easier to do what I know I can do.

"But the great thing about Julia is that each book has got a different subject and scenery, so it's not getting too boring."

Tiddler is available on BBC iPlayer from Christmas Day.