PRODUCER David Heyman had no idea what he was getting himself into when he began looking around for a children's story to turn into a family movie, on his return to London from a spell working in Hollywood three years ago.

During the search, his assistant suggested a book that was selling well in this country. He was soon on the telephone to the author to arrange lunch. She was reluctant to hand over her creation to film-makers, no doubt mindful that writers tend to be treated like second class citizens in the motion picture industry.

But she and Heyman hit it off, leaving her satisfied he'd treat her characters with the respect she felt they deserved. They shook hands. The deal was done. Heyman paid $700,000 for the rights to the first two books in the series. "Who could anticipate Harry would sell over 100 million copies worldwide?", he has since said.

For we are talking Harry Potter, as if there could be any doubt, considering the wave of Pottermania engulfing the media. Harry Potter and the Philosopher's Stone, to be precise. Or Harry Potter and the Sorcerer's Stone, to give the US title of the $125m film based on the first of JK Rowling's novels, which has been published in 47 languages in 200 countries.

The world premiere of the movie about the boy wizard is being held in London tomorrow afternoon, then opens in the US and UK on November 16. Like Star Wars, it is not a movie, it is an event that will be required viewing for children and adults. Some are tipping it to take more at the global box office than Titanic, currently the biggest grossing film of all time.

The top Warner Bros executive who said, with understatement bordering on the insane, "There's no question that there are large expectations attached to this project", was a rare example of someone in Hollywood keeping a sense of proportion about matters. There can be no doubt that the studio bosses, who need a smash hit after too many flops so far this year, must be on tenterhooks waiting for the reviews and opening weekend receipts.

Translating a hit book into film might not seem much of a gamble but, for every Dr Zhivago and The English Patient, there's a Bonfire of the Vanities which sinks without trace. The trouble with Harry is that his fans, not to mention Rowling herself, expect to see the characters and situations they know and love up there on the big screen. They don't want a Hollywoodised version that retains the name but jettisons all the aspects of the book that struck a chord with readers.

The Warner people have tried to be sensitive to this need to please the fans who they hope will return time and time again to see the movie. Repeat business is vital to achieve long-term box office success.

Steve Kloves, who wrote and directed The Fabulous Baker Boys, was hired to pen the screenplay. He admits that he was nervous about meeting Rowling for the first time. She had similar reservations, regarding him, as she says, "as the man who is going to butcher my baby". Happily, they found themselves on the same wavelength. During the writing process, Kloves contacted her for clarification about characters and events. "It's like having the bible of the story at other end of an email or phone line," he says.

Although Rowling had no official veto on decisions, the producers made her part of the creative process. She read drafts of the script and attended production meetings. Keeping her happy was important as there's nothing worse than a writer scorned. Remember how Anne Rice went public to criticise the casting of Tom Cruise in the film of her horror novel Interview with the Vampire? That she later changed her mind and gave him her blessing did nothing to stop the adverse publicity.

Rowling's insistence helped persuade Warner to make the Harry Potter film in this country with an English cast. Early rumours that Haley Joel Osment, the young American child star of The Sixth Sense, would play the bespectacled wizard were firmly denied. Besides, the inevitable countrywide search to find the screen Harry was some way away. Warner had yet to find a director.

The search began in earnest once Steven Spielberg announced he wasn't interested, opting to make AI Artificial Intelligence instead. Warner went as far as to audition directors. Chris Columbus, a fan after being introduced to the books by his children, fulfilled the producers' criteria. As well as writing credits on Gremlins and Young Sherlock Holmes, he'd directed young actors successfully in Home Alone and made hits like Mrs Doubtfire.

Daniel Radcliffe, who played young David Copperfield in a BBC tv series, was always his first choice as Harry but his parents had to be persuaded to let him undertake such a big project that would disrupt his teenage years. The adult cast was filled out with top British names including Maggie Smith, Robbie Coltrane, Alan Rickman and Richard Harris.

THE 130-day shoot was carried out in top secret conditions. A security blanket was thrown around the set whenever the film-makers ventured out on location. Several North-East sites were used - Durham Cathedral, Alnwick Castle and Goathland railway station - but most of the filming took place on the nine sound stages at Leavesden Studios, 20 miles outside London.

Warner Bros now has a 152-minute movie that hardly anyone outside the production team has seen, with Columbus working right up to deadline on the special effects needed to conjure up trolls, dragons, unicorns and three-headed dogs. Licensing deals with toy manufacturers and a soft drink company ensure the Harry Potter brand is everywhere.

Not only the British film industry but also our tourist trade, trying to reverse a drop in visitors following the foot-and-mouth epidemic and the US terrorist tragedy, will benefit. They are looking to Harry to work his magic by attracting people to the film's locations. Some already bear notices proclaiming proudly "Harry Potter was filmed here" and the British Tourist Authority is launching a map and website detailing the Harry Potter trail.

For Warner, the big question is whether one of the most successful book series in history, which has seen Rowling named the second richest woman in Britain after the Queen, can become one of the most successful movies ever. A long-running franchise, like Warner's Batman series, is something of which every film executive dreams. As much money, if not more, can be made from merchandising spin-offs as from selling cinema tickets.

No one doubts the first Harry Potter film will be an enormous hit. But how big? It's won the first round in the battle of the screen wizards by opening a month before the equally highly-anticipated Lord Of The Rings, and yesterday it emerged it has the biggest-ever advance bookings. But Harry must please audiences, and more importantly, the fans. If they're not happy, the future for films of the other books in the series looks uncertain. Whatever the reaction, at least one sequel will be made - production on Harry Potter and the Chamber of Secrets begins just days after the first film opens.

* Harry Potter And The Philosopher's Stone (PG) has its world premiere in London tomorrow. The film goes on general release on November 16, following previews in cinemas on November 10 and 11.

* Don't miss Harry Potter Week in next week's Northern Echo, which will keep both wizards and muggles happy. On Monday, we speak to a real witch about the right way to ride a broomstick, and a jumbo quiz gives readers the chance to be among the first to see the new film.