On a muggy May evening in 1997 I found myself on the steps of the Palais du Cinema in Cannes. The reason I had landed there was because a film of my first novel, The Dead Heart, was about to be screened. I'd yet to see the film, which was being shown under its new title, Welcome to Woop Woop. I'd yet to meet its director, Stephan Elliot, the man who had brought us Priscilla: Queen of the Desert. I had watched that film - and when the producer who had optioned my novel told me, some months earlier, that Stephan was on board as director, I wondered out loud whether his abundant talents for effervescence were suited to the very dark tale that I'd written.

"Don't worry about it," the producer told me.

The producer and I had already had something of a history together; she had hired me to write the first two drafts of the screenplay of The Dead Heart. She didn't like them, so she fired me, bringing in Michael Thomas, the writer of Scandal and Backbeat, to take over. Then, a few weeks before principal photography began, she rang me for a chat.

"We've made a few changes," she said, "but you're going to love it."

"What kind of changes?" I asked.

"It's best if you read the screenplay."

"Is it that changed from my last version?"

"It's really best if you read it yourself."

"Well, for example, how about that prologue where Nick is working on a local newspaper in Maine?"

"He's no longer in Maine."

"Oh, really. Where is he?"

"In New York."

"So he's working on a New York paper?"

"He's no longer a journalist," the producer said after a pause.

"So what does he do?" I said.

Another long pause.

"He's a bird salesman."

"What?" I said, thinking I was having some sort of LSD flashback.

"He's a bird salesman," the producer said again.

"You're kidding me."

"You sound shocked."

"Well, what do you expect? My journalist has become a dead bird salesman."

"No - he's a live bird salesman."

"Oh, that changes everything."

After reading the screenplay, I called a screenwriter friend in Los Angeles, an Oscar-nominated old pro in his 60s, and explained the situation. He laughed a chain smoker's corrosive laugh and said: "Cash the cheque and stay out of it. It's their movie now. And if it turns out to be a piece of shit, at least you'll always have your novel - and their money." I followed that advice.

And now here I was, on the step of the Palais du Cinema, meeting the director of the film of my novel for the very first time. Stephan Elliot turned out to be completely charming.

"Douglas Kennedy!" he said, pumping my hand. "Great to meet you! Hope you like the film! And, you know, I've really got to read your novel sometime!"

In the 11 years since The Dead Heart was published and filmed, I have written seven more novels. I have broken bread with producers and screenwriters and directors to discuss possible adaptations of my books. And along the way I have discovered one major truism about the movie business: the relationship between the novelist and the film-maker is never a straightforward one.

Consider the two French producers who wanted to turn The Big Picture - my second novel, about American suburban disaffection and an accidental homicide - into some sort of quasi-Marxist tract, set in Paris at the time of the Algerian war (they didn't get the rights).

Then there was the Hollywood studio guy who took me out to dinner in New York to tell me how much he loved the same novel, but had problems with a scene where the protagonist - about to go on the run for a murder he's committed - has to say goodbye to his young son for ever. The Hollywood guy also had an unfortunate (and very annoying) habit of poking me every time he made a point, and I remember being continually attacked by his right index finger as he explained how he would "fix" what was, to him, a morally questionable aspect of the book.

"Like it's one thing to kill a guy who's screwing your wife," he said, his finger landing repeatedly on my left tricep. "But to then run out on your kids? Like, no way, Doug. So how about this for an idea? Ben takes his son out for lunch, and hits him with the truth - that he's killed the man who's been doing the business with mom, and has to disappear for a while. 'But somehow, someday, I will come back for you,' he tells him. And the kid turns to his father with tears in his eyes and says: 'That's okay with me, dad. Follow your dream!'"

Fortunately, that Hollywood guy never had anything further to do with the book. But 20th Century Fox did option it, as did Renaissance Films in London after Fox dropped it. Renaissance closed, so for the past two years, Luc Besson's production company, Europa Films, has owned the rights. I've met with the producer, Pierre-Ange Le Pogam, and the director, Louis-Pascal Couverlaire, two or three times. I've also read the screenplay that Louis-Pascal and his writer have been working on since early 2005 - and approve of the way they have transposed the action of the novel from New York and Montana to Paris and Sicily. As I told Louis-Pascal during one of our cafe conversations in Paris: "If you can get the original spirit of the book on screen, I'll be pleased."

And, truth be told, that's all a novelist can ever ask of the people adapting his work for the cinema. Any writer who thinks otherwise really shouldn't consider putting his book up for sale - as he is certain to come out of the experience feeling, at best, very angry.

To a producer, a director or a screenwriter, your precious novel is nothing more than a starting point. No matter how much they flatter you during your initial meetings (that is, if you even get to meet them) and tell you the book is a masterpiece, remember: this is a seduction, and like all seductions, there's a great deal of fabricated talk. And the moment they utter the phrase, "We won't change anything," your bullshit meter should enter the deep red zone, as that statement is nothing more than a variation of the line: "I really will respect you in the morning."

The fact is, even if they do respect your novel they are going to do what they want with it for two basic reasons - one, because they can, and two, because they need to.

Apropos the first point, remember that selling your novel to a producer is somewhat akin to selling your baby to the first band of highwaymen who have trotted down the road, tempting you with a reasonable purse. From that point on, you have essentially abnegated all moral authority over how said baby will be raised in the future. And since few novelists ever have any contractual authority over how their book will be adapted, a producer and director can do whatever the hell they want with your novel.

Commercial considerations will often dictate how a novel is reconfigured for the screen - as will the idea that cinema-goers might not accept the same sort of hard realities that are contained in the original book. Look at Ang Lee's brilliant adaptation of Rick Moody's novel, The Ice Storm. In the book, an adolescent's accidental electrocution is followed by an unnerving scene in a car in which a deeply unhappy married couple (his neighbours) inform their children that they are about to divorce, the husband crying as he gives them this news. In the film, the husband (played by Kevin Kline) also breaks down in front of his family in the wake of the death of their neighbour's son. But then he is comforted by his wife (Joan Allen), a hint that this tragedy might be the beginning of a rapprochement between them.

Lee and his screenwriter (James Schamus) were probably right in opting for a more optimistic ending, because the rest of the film was infused with such a superbly rendered melancholy that to have piled on the bleakness at the end would have been too much to bear.

But Moody was fortunate to have a director and screenwriter in tune with his novel's sensibility. So often, if the film doesn't deviate wildly from the novel, then it fails to capture the book's innate sensibility (Brian de Palma's adaptations of Tom Wolfe's The Bonfire of the Vanities and James Ellroy's The Black Dahlia spring quickly to mind as films that botched the spirit of their sources).

Then again, all writers must understand that, being a visual medium, film uses an entirely different narrative vocabulary than a novel. It cannot afford to indulge in leisurely exposition or use a surfeit of dialogue to explain things. The words must always be married to the images, and the more reliance on things visual, the better. As such, a filmic adaptation of a novel is both a compression of the original narrative storyline and a visual reinvention of a literary tale.

Which is another way of saying, it's all one great crap shoot. As Clint Eastwood has shown, it's possible to make a very fine film out of a risible novel (The Bridges of Madison County) and an honourable failure out of a bestselling book (Midnight in the Garden of Good and Evil). But both films are remembered, first and foremost, as movies directed by Clint Eastwood, and not as adaptations of books.

So though you may be the author of the original book, the director becomes the auteur of the film. Which means that all you can hope for - besides decent remuneration for the rights - is an adaptation that somehow captures the essence of your novel. As such, novelists have to travel warily yet hopefully when they sell their books to the movies. If the film works, it cannot but help your reputation and your sales. And if it flops ...

Well, in the case of Welcome to Woop Woop, it got a tepid reception at Cannes, had very brief commercial runs in the US and Australia, never received a theatrical release in the UK, and occasionally gets a late-night screening on American cable movie channels. And whenever asked what I thought about the movie, I've always followed the advice of my jaded Hollywood screenwriter friend and said: "Mr Elliot has his film and I have my novel."

And, by the way, I did cash the cheque.

· Douglas Kennedy's new novel, Temptation, is published by Hutchinson