Scarlett Johansson was up to her old tricks, causing a stir again recently. But, it wasn't hard core partying or reckless rendezvous this time, it was on the set of her latest movie.

"Scarlett became very attached to the book," confides Philippa Gregory, author of The Other Boleyn Girl which hits cinemas this month. "So she caused a fair amount of consternation on set by coming to the screenwriter, Peter Morgan, with the book marked up with yellow marker pen saying 'You haven't used this scene at all, this is a great scene! It's got to be in!' It was very funny really."

Gregory is in London promoting the movie of her 2002 novel about Henry VIII and the two Boleyn sisters – Mary, the lesser known, who was the king's mistress, and Anne, who later became his wife.

"You've got to see fame and celebrity as a play. It's a rollercoaster that's about how good you are, how hot you are, and how much you earn one moment. And then the next moment, it's about someone else"

'A dream ticket' is how she refers to the pairing of Johansson and Natalie Portman who play both sisters. "They're just tremendous," she exhales. "Natalie is very seductive as Anne to start off with and then you see her almost break down under the pressure of trying to hold her position in court. Scarlett is incredibly tender, innocent and sweet. You can't help but love her. And Eric Bana as Henry VIII just smoulders. He's just so…so…so… tremendously attractive!" she giggles.

At 53, the same could be said of Gregory. Sitting in the breakfast room of The Berkley hotel, she radiates elegance in a make-up free way, with a blonde curly bob framing her soft face and huge, hypnotising green eyes. Much prettier than her picture.

She also has that attractive balance of youth, effervescence and maturity. Often witty and self deprecating, her answers are always honest, measured and wise. How has she remained sane in the mad world of famous writers and celebrity? Well, this is Gregory's first foray into Hollywood. Although The Other Boleyn Girl was adapted for the small screen by the BBC in 2003, she acknowledges that the film brings everything to a whole different level.

"To have your book made into a film is really one of the things that moves you from being a highly popular author to a worldwide author," she admits. "And that's where you want to be because it's tremendously hard to get to. Only very few books get optioned and even fewer films get made, so it's a big career jump."

She's curious about this new world of film but is resolutely approaching it with light hearted, anthropological fascination. "It makes you worry about stupid things like 'what will I wear' and 'I must get my nails done', things that shouldn't matter but are important in this type of world," she says, comparing it to day-to-day life on her farm.

"I think you've got to see fame and celebrity as a play," she continues. "It's not real life, and the last thing you want to do is start taking yourself too seriously. Actors have to have tremendous reserves to keep their feet on the ground. The attention is very flattering and addictive, but also very fleeting. It's a rollercoaster that's about how good you are, how hot you are, and how much you earn at any moment. And then the next moment, it's about someone else."

She recalls getting her first reality check from Morgan, whom she worked with as a consultant on the film script, when he called her from Los Angeles on the night the script was being optioned. "He said 'I'm calling to tell you that tonight, you are absolutely the hottest thing in Hollywood. Everyone is just crazy to get the script, they think it's going to be the film of the year. I'm telling you this so that you can enjoy it. Because let me absolutely assure you that, next week, they'll have forgotten your name'." She hoots with laughter at the memory. "Peter's a great bloke for that. He's another person that is absolutely not taken in by it."

But even if Gregory did enjoy the glitz and glamour of Hollywood (she admits liking 'nice' hotels and first class flights), she would never leave north Yorkshire - her earthly and spiritual home. As well as being an integral place for her writing, it's also where she lives with her second husband, whom she married in 2000, her children, ponies, ducks and swans.

"I pine for trees and grass," she gushes. "I get some kind of spiritual security and pleasure from being in the country, so although town is very nice for a couple of days, I need to be in touch with nature. It's how I feel free and comfortable and why I live on a farm."

"When I discovered Mary Boleyn, it wasn't that nobody knew about her, it's that nobody thought she was interesting. After she ceased being the king's mistress, she literally fell off the pages of history"

Success came quickly for Gregory. She wrote her first novel, Wideacre (1988), while finishing her PHD in eighteenth century literature at Edinburgh University. Seven highly praised books later came The Other Boleyn Girl which turned into an international bestseller overnight.

Though seemingly effortless, it actually took Gregory five years to research and write it, partly because she hadn't written about the Tudor period before and partly because Mary Boleyn simply did not exist beyond a few references in history.

"When I discovered Mary, it wasn't that nobody knew about her, it's that nobody thought she was interesting," she insists. "After she ceased being the king's mistress, she had literally fallen off the pages of history. Now, whenever I see a play or read a book about Henry VIII, if Mary is in it, I know they've read my book. Because nobody else has written about her."

It's taken another five years for the book to be turned into a move she's 'really pleased' with. But Gregory hasn't been sitting around waiting. As well as publishing three books since then, her latest, The Other Queen, about Mary Queen of Scots, will be out later this year.

"The thing that still matters most to me is writing a good book," admits the former journalist. "How the book is received, I can't control. But what I can control, and what I love and take pride in, is writing the best book I can with good research and which is well crafted. The rest is the cream on the cake."

Gregory has obviously had enough cream. She's getting noticeably itchy for home after a long few days spent in the city doing publicity, meeting her fellow 2008 Orange Fiction judges, and even fitting in a little shopping. Within hours, though, she'll be back to spiritual equilibrium with her ducks, her family and writing more bestselling novels, no doubt.

By Barbara Walshe