Coming across a surprise success such as Louis de Bernieres's novel, Captain Corelli's Mandolin, must be what agents and publishers dream of over their morning coffee. Expectations at its publication were modest, but then, largely through word-of-mouth publicity, it went on to become a book that welded itself to the best-seller lists for years, inspiring dewy-eyed devotion in its readers.

A foregone conclusion, then, that someone, somewhere, would make a film out of it. But it wasn't the easy step to money and success it might appear. Even more dramatically than usual, the audience of Captain Corelli's Mandolin will bisect itself instantly, one part comprising those who have never read the book, one encompassing those who have. The novel's cult followers, some of whom have read it over and over, may be aghast at the transposition of its narrative into celluloid.

The original, published in 1994, tells in wry and lyrical style the story of the people of Cephallonia (one of the Ionian islands off the west coast of Greece) trapped in the Italian occupation during the second World War. Iannis (John Hurt), a widowed doctor, and his intelligent and wilful daughter, Pelagia (Penelope Cruz), are at the centre of this Greek community. An unlikely love affair between Pelagia and a musical Italian officer, Captain Corelli (Nicolas Cage), provides the nucleus for the drama, humour and suffering in the narrative.

For the film, which must distil the essence of the book in its relatively short running period, difficult choices about characters and their interaction had to be made. Neither director John Madden nor scriptwriter Shawn Slovo could afford to be too squeamish about tweaking the book. Their alterations and creative urge resulted from a reverence for the material of this much adored novel.

"In some ways I was anxious," admits Madden, "and in some ways, I wasn't. Any book you have a kind of respect, admiration and affection for makes you very aware of your responsibilities when you approach it as an adaptation. But an actual adaptation is a form of flattery, it's the best way to pay tribute to the book. To be effective, the film could not have trod the same steps as literature."

Shawn Slovo took on the task of screenwriter before the extraordinary success of the novel swept in. "So I didn't anticipate the amount of baggage that comes along with making that kind of choice: to adapt something that so many people have read and loved." But even before the novel became a best-seller nothing came easily. Slovo laboured over the screenplay for five years. "It was an incredibly difficult book to adapt because of the size of it, scope of it, scale of it."

Many things about the book quickly proved unworkable in a screenplay. The original ending, in which Pelagia and Captain Corelli meet again having been parted for over 40 years, had to be jettisoned. And Madden "shed bitter tears" when he realised he couldn't do justice to the heart-wringing story of Carlo Guercio (Piero Maggio), the courageous soldier who saved Corelli's life. A character for whom Madden originally had more elaborate plans, "he became a silent presence, although in the book he was a very eloquent presence". But allowing this magnificent character to dwindle into one line, though painful, was "one of the things that you have to be grown up about and make your choices about".

The role of Pelagia's rejected fiance, Mandras (Christian Bale), alters radically to fuel the plot. He transforms from a character whose story illustrates how radical political movements chew up and psychologically dismember unwitting, innocent volunteers to an element adding increased pressure, hindering the development of Captain Corelli and Pelagia's romance. As Madden explains, "Corelli was living in Iannis's house and falling in love with this woman in an environment without any emotional obstacle at all. I felt that for any story to unfold on the screen it was essential that there was obstacle and conflict, and Mandras seemed that key."

Slovo echoes this concern. "It makes the love story between Pelagia and Corelli more exciting. For me the main draw in the film is the love story. There are two people who are not looking to fall in love - I mean, he certainly isn't, she's got a fiance - and I think it's that kind of reluctance, that coming together in spite of the obstacles, that makes a good love story, don't you?"

The love story alone was not what fired her enthusiasm for the film. And "it wasn't the popularity" of the book, either, insists Madden. The project was well underway before the novel became such a phenomenon. "The book has a wonderful range of tones to it," he explains. "Funny, harrowing, it approaches the second World War from an unusual and unexamined perspective. It's concerned with a war that's not taking place. And then people are plunged into cataclysms of unimagined brutality. The contrasts of the book, its scale, the intimate story set within massive conflict . . . it's a story about love and survival."

Unusually, Captain Corelli's Mandolin was shot entirely on location on Cephallonia. Madden spent five months on the island, four of which encompassed the actual filming. "It was very, very gruelling to shoot - and absolutely exhilarating and wonderful. Not just because of the setting, but also because of the intensity of the experience. There was something about telling the story in the place where you knew it had happened. Every location we went to, the reality of the fictional situation we were in lay under what we were doing."

Despite the effort involved, Slovo "loved everything about the film. I'm terribly pleased with the outcome." Had she no qualms about taking on the colossus of this book and its formidable reputation - not to mention the reaction of its stalwart fans? "I come from a family who have always encountered obstacles. I like a bit of danger, so I like the challenge."

Although die-hard fans might quail at the thought of anyone tampering with "their" book, many of those who haven't read the book expressed admiration for it, some going so far as to admit to a wish to have a(nother) go at reading it. And, according to Madden, this is what the relationship between the genres should accomplish. "I hope that the people who see the film feel inspired to go back to the book and read it." As he hopes that those who have read Captain Corelli's Mandolin will be keen to watch its adaptation.

For any remaining sceptics, Madden offers this recommendation: for films based on books, you really need to see them twice. The first time establishes the film as the separate and unfamiliar entity - "an emotional twin sister", according to Slovo, "in that it should acknowledge its source but should be able to exist on its own". The second time you are prepared for its difference, and can take the film on its own terms.

And yes, Nicolas Cage did play the mandolin himself.

Captain Corelli's Mandolin will be on general release from Friday