LONDON -- At the first test screening of "Harry Potter and the Sorcerer's Stone" -- which took place, believe it or not, at Chicago's 900 N. Michigan theater a few weeks ago -- director Chris Columbus got a flavor of the reactions he should expect from some pint-sized critics.

"I sat behind a kid at the screening who kept shouting things at the screen, not loudly but to his mother, and I was hearing everything," Columbus recalled. "We were in the Leaky Cauldron (a wizards' pub in the movie), and Harry walks in, he's meeting the first person who recognizes him, and the kid says to his mother, 'Dedalus Diggle is next. Dedalus Diggle,' who is this other guy who meets Harry (in the book), which I shot but I cut.

"And then we skipped over Dedalus Diggle, and he said, 'Dedalus Diggle wasn't there.' And he kept doing that." Filmmakers who adapt a popular book always must risk the wrath of its dedicated fans, but with "Harry Potter and the Sorcerer's Stone," which opens Friday, we're talking about an especially large and literal-minded group -- i.e., mostly kids. J.K. Rowling's four "Harry Potter" books have sold well more than 100 million copies worldwide, and many readers will be pitting a movie against their imaginations for the first time.

"Children as you know are terrible sticklers for detail -- you know that if you ever bought the child the toy that is not exactly the one that they wanted," said Robbie Coltrane, who plays the lovable 8-foot-tall Hagrid, the groundskeeper at the wizard school Hogwarts. "I got a letter from a woman the other day. ... She said, 'We're so glad you're playing Hagrid. Of course, it's going to be very, very difficult to get that blend between scary and innocent humor, and millions of children throughout the world are relying on you.' So I thought: no pressure there, then." The pressure to be faithful comes not only from the book's audience but also its author. Rowling sold the rights to the projected seven-book series to producer David Heyman (who last oversaw the 1999 cannibalism tale "Ravenous") and Warner Brothers for a reported $700,000 after six months of negotiations that, from the producer's point of view, was far less about money than the commitment to preserve Rowling's vision. (Heyman said he signed the deal the day before "Harry Potter and the Sorcerer's Stone" was released in the U.S.)

"We wanted to be faithful because the book works and because I made that promise to Jo (Rowling)," Heyman said.

Longer than average

Rowling considers the promise kept, saying before entering last Sunday's "Harry Potter" world premiere at the Leicester Square Odeon, "I am happy with the film. It's very, very faithful to the book, and that's obviously the most important thing to me."

That faithfulness was a recurrent theme of a day of interviews with many of the movie's main players, excluding Rowling and the apparently travel-averse screenwriter Steve Kloves, who canceled his appearance. The principles were gathered at Knebworth House, a 15th Century mansion outside of London that Warner Brothers staffers had dressed up with Hogwarts props.

The movie, called "Harry Potter and the Philosopher's Stone" in Britain (as was the book), runs 152 minutes, about an hour longer than your average kids' film. But such a length was necessary, Columbus said, to cram in as much of the book as possible.

"(The studio) wanted the film to be two hours long," the director said. "I said it's never going to be two hours long if we want to remain faithful to the book. You'll have people burning the prints of the film. Kids are reading a 700-page book ('Harry Potter and the Goblet of Fire'). In the '60s people sat through (2=-hour-plus) movies like 'Oliver!' and 'The Sound of Music.'."

Fidelity to the printed source, however, is not necessarily a hallmark of classic film adaptations. "The Wizard of Oz" not only trimmed numerous sections from L. Frank Baum's immensely popular "The Wonderful Wizard of Oz," but it also made the kind of additions that would have gotten Columbus clobbered. For instance, in the book Dorothy wears silver, not ruby, slippers; the Wicked Witch of the West doesn't appear until after Dorothy and friends already have visited Oz in the Emerald City; and the Scarecrow, Tin Man, Cowardly Lion and Wicked Witch have no real-life counterparts in Kansas.

'Don't mess with it'

The movie of "Harry Potter and the Sorcerer's Stone" retains the book's basic structure: Harry, orphaned and living with his mean relatives the Dursleys, learns he's a powerful wizard, takes off for wizard school and, with help from his friends, does battle with evil.

"As a fan I loved the book, and I wanted to bring that to life," Columbus said. "How could you be egotistical enough to go in and say, 'OK, this is the way 'Harry Potter' should be done'? 'Harry Potter' is done really well so don't mess with it too much. At least my own kids were telling me that."

In fact, Columbus got the job over Terry Gilliam ("Brazil"), Ivan Reitman ("Ghostbusters") and Brad Silberling ("City of Angels") not only because of his box-office track record with family-oriented films ("Home Alone 1 and 2," "Mrs. Doubtfire") but also thanks to his detailed presentation about how he would translate the book's elements to the big screen.

Meanwhile, alternative ideas were striking fear into the hearts of Harry fans. Steven Spielberg reportedly considered making a computer-animated adaptation with Haley Joel Osment providing Harry's voice. Another suggestion was to move Hogwarts to the U.S.

"People were suggesting, why not cast an American Hermione, why not combine the first two movies -- all horrible, horrible ideas because I felt this was a movie that had to be an all-British cast," said Columbus, who promised as much to Rowling. "I got a call from Rosie O'Donnell saying, 'What do you mean? I could do a great British accent. I could play Mrs. Weasley.' And I said, 'No, no, it's an all-British cast.'."

The lingo also remained British. "We did have an argument with the studio about the American version changing 'bogeys' to 'boogers,'." Columbus said.

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