Beloved children's author Roald Dahl lived a life of paradox, cultivating the image of a kindly creator that belied his reputation as a short-fused egotist, his first-ever authorized biography reveals twenty years after his death.

Celebrated publicly as an inventive writer who spun fantastic tales sitting in a large armchair in his garden shed, Dahl is also exposed in "Storyteller" as a curmudgeon who just as willfully fabricated details of his life.

Dahl wrote books that are popular to this day, ones such as "James and the Giant Peach, "Charlie and the Chocolate Factory," and "The BFG," in which the eponymous hero, the Big Friendly Giant, blows dreams into children's bedrooms as they sleep.

But there were times he morphed into a Big Unfriendly Bluffer as he persistently rewrote history and angered associates, friends, and publishers with provocative remarks, biographer Donald Sturrock told in a telephone interview.

Sturrock first met the author as a young BBC producer in 1986 when he proposed shooting a documentary about him and came to consider Dahl a friend.

He wrote the book with full access to the writer's unpublished letters and unexamined personal files upon the request of Dahl's daughter Ophelia.

In the days that preceded the release of the biography, several book reviews seized on anecdotes that focused on Dahl's explosive bad temper and his flagrant disregard for the truth.

While Sturrock describes the man he knew as "terrific fun," he also confesses he often wondered why Dahl had earned the reputation of having a short temper. 

Shortly after he died, however, Dahl's second wife Felicity told Sturrock the bad impression he had made upon landing on the author's doorstep.

"Oh Christ, Lic, they've sent a f--king child," he groaned to his wife.

Flagrant falsehoods

Some of the falsehoods Sturrock uncovered were innocent enough. Dahl said his paternal grandfather Olaus was a "prosperous merchant" but records revealed he was a butcher. On one occasion, Dahl taught Sturrock how to shuck his first oyster, using what he said was his father's wooden pocketknife that had travelled around the world with him since he was but a schoolboy. Years later, Sturrock found out that was just an old knife he had grabbed from the kitchen.

Dahl also claimed that when the war was declared in 1939 and he was working for the Shell Petroleum Company in Tanganyika (modern-day Tanzania), he had to round up Germans. Dahl said an African guard shot one of the Germans when they shoved a pistol in his chest.

The truth was that all of the Germans complied.

But Sturrock told in a telephone interview that people misunderstood Dahl's tendencies to over-exaggerate and shock his friends and associates. He pointed to a line in "The BFG" in which the Big Friendly Giant says what he thinks and what he says aren't necessarily the same thing.

"I know exactly what words I am wanting to say, but somehow or other they is always getting squiff-squiddled around," the BFG tells Sophie, the young girl he snatches from an orphanage.

This was also true of Dahl, Sturrock said.

"While he could be bad tempered and hurtful, I don't think he intended to hurt very often and I think probably his greatest weakness in that regard was his ability to say sorry. If he had said sorry more a lot of people who he had hurt would have forgiven him," Sturrock said.

Sturrock wishes he could have conveyed "twinkle in his eye" or the "chuckle in his voice," when Dahl made some of his more controversial, outlandish statements, he said.

Facts bored Dahl

"Dahl was such a lover of fiction that he was often unaware of when he was embellishing it. It was a completely natural thing to him to alter the truth," he said.

But Sturrock believes it was Dahl's childhood demons rather than his penchant to embellish facts that influenced his writing.

"At a very early age he learned that life could be pretty cruel and that human beings survived those blows," Sturrock said.

Dahl was born in 1916 to Norwegian parents, Harald and Sofie Magdalene, in Wales. He was the only son among their four children. But tragedy struck early.

His sister Astri, four years older than him, died when he was three. His father died soon afterwards. As a young boy, he was sent to several boarding schools in England, where he was homesick and miserable under a strict British public school system that had no qualms about caning its students.

"And this quiet sensitive soul was hardened up. And then as a young man, he fought in the war and shot people down and killed people and had all the neuroses associated with that. All those things weighed quite heavily on him and they played a big part in him becoming a writer," Sturrock said.

Then as an adult, Dahl's baby Theo suffered severe head injuries when a car drove into his stroller; two years later, he lost his oldest daughter to encephalitis when she was 7; and then three years after that his first wife, Oscar-winning American actress Patricia Neal suffered a debilitating stroke. Dahl took control of her rehabilitation with a strict regime and within years she had returned to acting.

Despite his help getting Neal back on her feet, Dahl was not a faithful husband.

He once told readers of Ladies' Home Journal that quick, physical affairs were best because what made relationships work was 70 per cent sexual attraction and 30 per cent respect. He bedded Martha Gellhorn, while she was still married to Hemingway, actor Tyrone Power's wife Annabella, and cosmetics queen Elizabeth Arden. On the latter affair, "It didn't last long," he told friends, "but it cured my spots."

As time passed, his skeletons aged and acquired more flesh, Sturrock said.

But the notion that life was cruel never abandoned him, Sturrock said.

In one of Dahl's "ideas books," in which he jotted down thoughts as they came to him, Sturrock discovered one full page of cruel things that people to do animals before they eat them, including scalding live animals in hot water, or the French force-feeding geese to make foie gras.

Life before Harry Potter

Dahl's dark side, in which he is preoccupied with greed, revenge, and the sinister underbelly of human nature is evident not only in his adult literature but also in his children's books.

"His children's villains are as villainous as you can get, really," Sturrock said.

Today, even decades after his death, there is still a gaping hole in the children's literature field, Sturrock agreed.

"If there was such a thing as a children's writer, he was it. ... Strangely, children's fiction has acquired a certain respectability that was not true in his own lifetime," he added.

Without Dahl, the Harry Potter series may never have existed as we know it, Sturrock said.

"He did liberate something in children's writing that moved it forward from being a rather-protective, almost-Victorian, tradition of adults writing for children and writing the books they think children should read and enjoy rather than writing from a child's perspective.

"... And lots of people rushed in to fill the landscape he created."