LIKE most parents I’m indebted to Roald Dahl. Over the years he has effortlessly reduced me and my two children to fits of laughter, time and time again. Nevertheless, standing in the crowd at the opening of the Dahl Museum the other day I felt a deep sense of relief that we hadn’t actually met. You see I have a beard. Nothing wrong with that, unless, of course you’re Roald Dahl and you hate beards, in fact any facial hair. And furthermore with a passion that is disproportionate, ferocious even. Remember the hairy Twits, surely Dahl’s most unappealing characters. Until now I’d always considered this a work of fiction.

Then in the course of making our Imagine film about Dahl, John Bridcut, the director, and I came across a long-lost home movie featuring Dahl, the father of the bride, proposing a toast to the hapless, hairless (in the facial department) bridegroom. Suddenly, uncannily, fact and fiction collided.

“The thing to do if you want to judge a man immediately and with absolute accuracy,” proclaimed Dahl to the wedding party, “is to look him in the ear hole. If you see tufts of hair sprouting like grass — a rich pubescence as we ear-hole experts call it — you know for certain the fellow’s a bounder . . .” And on he went.

“Oh, he was such a child,” his daughter Tessa recalled indulgently as she listed the countless occasions on which “Mouldy”, as his grandchildren dubbed him, had misbehaved. At parties, in restaurants, at home. He called it “jokiness” and wore it like a badge of honour. It was, he claimed, a professional asset.

“When you’re old enough to write a book for children,” he confided later in life, “you have lost all your jokiness. Unless you are a kind of undeveloped adult and still have an enormous amount of childishness in you I don’t think you can do it.”

Dahl never lost that jokiness and of course children got the joke. They were the customer. He aimed his books directly at them. He wanted to go over the heads of parents and librarians, the traditional guardians of children’s literature. In Matilda he makes the parents so appalling that the five-year-old heroine ends up negotiating with her own parents to give her up for adoption.

All his stories have a subversive streak. They belong to a folk-tale morality with a black and white sense of good and evil. He saw himself on the side of the small child, surrounded by giants — the enemy, he called them, even though he himself was the biggest (6ft 6in in his slippers, incidentally) and (when he chose to be) the friendliest of the lot.

In real life, too, he was far more at ease with children than with adults. He was irresistible and intimidating in equal measure. Quentin Blake, Dahl’s illustrator, recalls an encounter with two eight-year-old boys. “What do you do if someone doesn’t like your work, Mr Dahl?” they asked the big man. “We hit them,” he replied.

In life and literature, on and off the page, he kept them in suspense. It’s part of the reason why it was hard to tell where his storytelling ended and reality began. That’s how he became a children’s author in the first place.

Staying at the Dahls’ house meant being woken up in the middle of the night and being taken down the lane in your pyjamas in the dark — then standing in the tunnel and waiting for the train to come . . . Dahl was like the BFG blowing dreams of adventure in through their bedroom windows. “I can remember,” his granddaughter Sophie recalls, “going very wide-eyed and thinking, really, are you telling the truth, really, really? I just never knew.”

In so many ways Dahl was an exemplary father, but he remained inscrutable even to close relatives. In his children’s words he wasn’t “a hugger and kisser”. For most of his life he found it difficult to be demonstrative, yet he was the rock at the centre of family life.

It was Dahl’s extraordinary determination, his wilfulness and his black humour that enabled the family to survive disasters that could so easily have destroyed their lives — though the years of tragedy took their toll. In 1990 he was in and out of hospital with leukaemia but he remained the “undeveloped adult” to the last. He was caught smoking one of his large cigars, hanging out of the hospital window with a porter on each foot. He promised not to do it again, and did the same thing the next day. But that was Roald Dahl.

• Fantastic Mr Dahl, Alan Yentob’s portrait of the author in his Imagine series, is screened on BBC One tomorrow at 10.40pm. The Roald Dahl Museum is at Great Missenden, Bucks (01494 892192)