She wrote a novel, her 15th to date. It wasn't out yet so she wasn't even thinking about reviews and all that jazz. The proofs were being hawked around in the usual fashion. Then a chap in the film industry calls up from his car in the United States and says he loves it and wants to make it into a movie. His name is Spielberg, Steven Spielberg. Her name is Deborah Moggach. The novel's name is Tulip Fever.

Moggach is sitting in the Shelbourne Hotel in Dublin, about to start nibbling - eating is too earthy a word for such an elegant-looking woman - on a plate of seafood. "Two of my favourite things - eating seafood and talking about myself," she says gravely. She's done a lot of talking about herself lately, since news of the Spielberg interest became known.

English-born Moggach's life story veers a bit on the fictional-sounding side. She grew up in a house where both parents were writing and painting. Her father, Richard Hough, wrote "oh, at least 100 books. Adventure, children's, popular naval history, a biography of Mountbatten. That sort of thing." Her mother, Charlotte, was an illustrator.

Deborah Moggach grew up taking such wide-ranging creative industry entirely for granted, which is probably one of the reasons she has had such a successful and prolific career. Apart from her novels, some of which she has adapted for television, including Close Relations for the BBC, she's also written a stage play, Double- Take, and won a Writers Guild Award for her television adaptation of Anne Fine's book, Goggle Eyes. Her work has made her very wealthy.

On the personal side, she left her husband of 14 years, with whom she had two children, to be with the cartoonist Mel Calman. She was with him until he died - 10 years later, of a heart-attack while they were watching a film in London's Empire Cinema. She now lives with a Hungarian painter and designer, Csaba Pasztor, who is 15 years younger than her. He has transformed the Hampstead house they share into a baroque glory hole of found treasures. It is all, as one British journalist observed recently, "very exciting".

Perhaps not quite as exciting, however, as Tulip Fever has turned out to be. Fever is certainly the right word. Rights have already been sold in 14 countries. Moggach wrote the novel in four months: "terribly, terribly fast. Usually my books take a year or so." It's set in 17th-century Amsterdam, and tells the story of a young girl, Sophia, married to a much older man, Cornelis. He decides it would be a good idea if they were immortalised in a portrait. Enter Jan, the painter, who falls in love with Sophia, and she with him . . . Enter deception.

The tulip fever of the title refers to the 17th-century obsession with trading in these bulbs, where huge sums of money were paid for new bulbs. "The nearest thing you'd get to that these days is Internet trading; they call it dealing in the wind," Moggach says.

The novel was inspired by Moggach's fascination with Dutch painters of the period, among them, Vermeer, van Rijn, de Hooch, and Maes. She thought that the rich, dim interiors of the rooms these painters created, with their subjects reading letters, writing letters, watching for someone unseen through open windows, whispering on stairways, seemed to invite fictional exploration. There are 16 colour plates interspersed through the book.

She is intrigued to hear about Paul Durcan's two books of poems, Crazy About Women and Give Me Your Hand. Both are based on paintings: one from the National Gallery's collection here, the other from London's National Gallery. "Oh, I'm going to go and look at the Vermeer again this afternoon," she says. Vermeer's Lady Writing a Letter With her Maid, which is one of the plates in the book, is in the National Gallery here. "I'll look out for those two books; I hadn't heard about them."

At the time Spielberg made the call from his car, another studio, Mirimax, was also voicing interest in the book. Tough luck. "Because it was Spielberg, we went with him. It was just so irresistible," she explains. She flew out for a meeting with him in Hollywood the week of the Oscars. "Quite a heady time. There was the stretch limo and all that. I like glitz. I'm a sucker for it."

She brought several books of reproductions of Dutch paintings with her as a gift for Spielberg, so he could get a visual sense of the kind of interiors she had in mind when writing the book. Their initial meeting was at a private house, and what struck her most about it was "how casual and friendly and informal it all was".

The next day she was shown around DreamWorks, Spielberg's studio. "An amazing place. It's all huts and Navaho blankets and drapes everywhere. Very ethnic looking, right in the middle of Universal Studios, full of goodlooking, smiley people striding about. It didn't seem like a film studio at all. Too friendly for that." Not that friendly, though. Moggach wanted to write a newspaper diary piece about the experience, but DreamWorks were "very funny" about it, and said no.

Spielberg is very keen to make the movie, which, all being well, will go into production next year. Period films, as Moggach points out, are hot. "And since it's Spielberg, a director who also owns his own studio, it's far more likely that the film will get made." Still reeling from the whole experience, she is currently finishing the first draft of the screenplay.

She is now in the extraordinary position of wondering which actors would play her characters best. "He wants English or European actors, but I think Claire Danes (who appeared in Romeo and Juliet) would be wonderful. Or maybe Gwyneth Paltrow. Except they're a tiny bit American. Well, they are American . . . "

If the film gets made, she will make an enormous amount of money. "I hate buying clothes. I don't buy female things much. I like buying things like old balustrades for the house." She looks down at what she's wearing: black velvet trousers and a simple pair of slip-on shoes. "I was wearing these trousers when I met Steven," she announces.

When she arrived back from the United States, her children, Tom (23) and Lottie (21), had decorated the house with tulips and the banner-like printouts of the faxes DreamWorks had sent in between meetings and deadlines. Moggach would like to make enough money to "see my children settled": they both still live at home.

"What I really want to buy is a walled vegetable garden, but it's a bit difficult to find one in central London." She's mad about gardening, and was thrilled by the gift her sister gave her for her 50th birthday last year. "She gave me five of the oldest tulip bulbs in the world. They date from 1680. She sent off to a specialist in Holland for them." The flowers have red and white stripes, and they opened for the first time when Tulip Fever got to the proof stage and Spielberg read it.

Asked what she bought to celebrate the sale of the film rights, she replies "manure for my garden", as she daintily spears another mouthful of smoked salmon.

Tulip Fever, by Deborah Moggach, is published by Heinemann at £14.99 in the UK