Tracy Chevalier's novel about the Dutch artist's best-known work has been turned into a film. She tells Arminta Wallace about it.

An enigma wrapped in a mystery. You could say that about any great artist, of course, but the life and work of the Dutch painter Jan Vermeer are more enigmatic than most. He is reckoned to be one of the finest painters of all time, yet his fame rests on a very slim body of work: 35 paintings, or 36, depending on who's doing the counting. Little or nothing is known about his life. "Son of a silk worker," declares one biographer confidently; "son of innkeepers," insists another. For two centuries after his death, in 1675, his paintings were ignored or attributed to other artists; these days, art students spend hours deconstructing them, trying to figure out just what it was Vermeer did with light and space that turned a series of images of cosy bourgeois burgherdom into something approaching the sublime.

Stand in front of a Vermeer painting and you feel like a voyeur. You will, most likely, be looking through a door frame, or from behind a drawn-back curtain, at a domestic interior featuring one or two people, usually women. The scene will be one of ordered calm that also seems to glow with dramatic significance. Vermeers invite speculation. They suggest stories. National Gallery, London: Girl Standing At Virginal. Why is she standing? Not to impress the viewer, that's for sure - the stance is more awkward than graceful. National Gallery, Dublin: Lady Writing A Letter With Her Maid. What is she writing about? It isn't an easy letter, to judge by the crumpled ball of paper on the floor at her feet and the expression on the face of the waiting maid, who has clearly been gazing out of the window for some time.

The most famous Vermeer enigma is a matter of identity. Who is - was - the girl in the painting entitled Girl With A Pearl Earring? His daughter Maria, say the art historians. No way, says Tracy Chevalier, whose 1999 novel brings the painting to extraordinarily convincing life. But she admits it was the enigma that attracted her to the subject in the first place. "If more had been known about Vermeer I wouldn't have written the book," she says. "If I had known who the girl was there would have been no story. People often say to me: 'Why don't you write a book about Rembrandt?' And it's true he had a really soap-operatic life, but so much has been written about him that you might as well just read a biography. Whereas with Vermeer there's nothing left of him except his paintings. A few details here and there, but nothing that can tell you what he was like as a person."

How can Chevalier be so sure, then, that the girl in the painting is not the artist's daughter? "The painting seems to me to be very erotic, though it's a very understated eroticism," she says.

"One of the conventions of Dutch painting at the time is that if a woman has her mouth open she's meant to be sexually available. Actually that's true even today, in photographs. There's a very big difference between mouth open and mouth closed. I'm not sure Vermeer meant that with her, actually. But everything about her - her skin, her eyes - is so liquid and so sensuous, and the look she's giving him is full of desire. Which is what makes me think it's not his daughter."

On the other hand, she adds: "He managed to capture several expressions at once, so you never quite know what she's thinking. I've written a whole book about her and I still don't know what she's thinking." The ambiguous relationship between painter and painted is what provides dramatic tension in the book Girl With A Pearl Earring. Did Chevalier worry about what might happen, then, when the time came for her novel to be made into a film? She nods grimly.

"Restraint," she says, "is very unusual in Hollywood. Which is partly why I wanted it to be made by a British production company. One of the first questions I asked Pathé Films was: 'Are you going to have them sleep together?' And they said no."

Still, from the minute detail of the page to the broad sweep of the screen is quite a leap. When she was researching Girl With A Pearl Earring, Chevalier studied 17th-century Dutch art to the point at which she can talk happily about, say, the light source in Vermeer paintings habitually coming from the left-hand corner, so the shadow of the painter's hand would not fall over his canvas. She also took a painting class - with, she says wryly, mixed results. "When we did a life model, mine ended up looking like Prince Charles even though the guy didn't look anything like Prince Charles to begin with. I don't know how that happened. But it made me understand how hard it is. In the end I threw all my paintings away.

"But what I learned from studying Vermeer's paintings is that less is more. I figured that if I was going to write a book about a Vermeer painting, then it should be a little bit like a Vermeer painting. Small brush strokes that are not flashy; no verbal fireworks; prose that you don't actually notice."

Sounds like a recipe for big-screen disaster, especially when you add Chevalier's own plot summary: "Nothing happens." But the film is visually sumptuous, for which the credit must go to the director Peter Webber, a former student of art history, and the cinematographer Eduardo "Wings Of The Dove" Serra.

"It is very beautiful," says Chevalier. "It's like a whole series of paintings, and not just Vermeers; you can spot Rembrandts in there, and \ de Hooch and others. The director told me that, when they were filming, the cinematographer would say: 'How shall we light this today? Let's go for a Rembrandt look.' "

Chevalier is delighted by the way the American actress Scarlett Johansson brings the character of Griet to life, often without the benefit of words. There is a breathtaking moment when Johansson becomes the painting. "People say Scarlett looks like the girl in the painting," says Chevalier. "She doesn't. But by the end we believe that she does, because she inhabits the role so completely. Part of it is that the pose itself has become so famous - it's like Vermeer invented that three-quarters gaze over the shoulder."

As a writer, isn't she incensed that they changed not only the end of her book but also the beginning? "I accept that a film doesn't have to be by the book," she says. "It has to tell a story differently. At the end the book skips forward 10 years; for a film to do that is a whole different ball game."

As for the beginning, it fell victim to a 21st-century curse: celebrity. One of the most visually appealing aspects of the film will, for many, be that in the role of Vermeer it features Colin Firth in an elaborate wig, striding around and looking anguished. Very easy on the eye. Chevalier chuckles.

"Even in that wig, well, you know, that's one of the reasons they changed the beginning. Originally it was filmed the way it is in the book, where the Vermeers go to visit Griet while she's chopping vegetables in the kitchen. But when they got to the cutting room the director said it would ruin the mystery, because everybody would just go, oh, look, there's Colin Firth as Vermeer. So you don't get a proper look at his face until about 20 minutes into the film."

Enigma wins after all.

Girl With A Pearl Earring is due to open on January 16th