Josephine de La Baume: Mark Ronson's number one

Josephine de La Baume's oval, big-cat eyes, which she has outlined with black liquid eyeliner, are looking heavenwards. She is trying to describe how she felt when she met her fiancé, Mark Ronson, three years ago. 'It was almost like a thunderstorm, it was so strong. When you fall in love, you know very fast that it's gonna be huge and very important, and at first maybe you try and fight it a bit for a few days, but then it overtakes you even stronger, you just can't resist it'

Being in love (or, as her delicious French accent has it, 'in loff') has made her 'more generous and stronger and more confident in everything'. Ronson, 35, proposed earlier this year on a windswept beach in New York. 'It was freezing, our noses were dripping and we had tons of clothes on, I was wearing gloves,' she says, admiring her square-cut yellow diamond ring. 'It was completely unexpected, a beautiful moment.' They are marrying on 3 September in the South of France, near her parents' home in Aix-en-Provence. The guest list, set to include Lily Allen and most other music stars you can think of, is going to provide her with the fun of doing a seating plan - 'I'm going to do some matchmaking' - and Zac Posen is designing the dress. 'I keep telling Zac, "Make it smaller, I'm going to be much skinnier for my wedding," but he thinks that's wishful thinking.' She is interrupted by her mobile phone. 'It's an "unknown number". That means it's my mum, calling about the wedding. I'm wise to that now.' She slips the phone into her bag.

Joséphine, 26, is your archetypal Gallic dream-girl: a TV, film and stage actress; a singer; an uninhibited underwear model; a liberal, bien-pensant aristocrat. She is soon to be seen in the film adaptation of David Nicholls' One Day, playing Marie; as a secret agent in the Rowan Atkinson spoof Johnny English Reborn; and in the Julian Fellowes TV drama Titanic. She is about to release a single with her band Singtank on the Warner France label, co-performed with her brother Alexandre, and produced by Nellee Hooper. And she is currently to be seen as the face of the Leon Max fashion label and sauntering around in suspenders in a nouvelle vague-style short film for Agent Provocateur. 'It says, "Look, I'm a woman, I'm happy with my body, whether it's perfect or not." I don't think it objectifies women, in fact I think it's quite feminist.' And as for the aristocrat thing, well, her father is a baron, 'but because we live in a république, it doesn't really mean anything to me, it's more or less irrelevant,' she smiles. 'I don't have three châteaux.'

She arrives for our 11am interview in sky-high wedges, comme toujours. 'I'm a master of heels. I'm used to them. I'm tiny so I wear them all the time. Apart from to the beach. That would be slightly psycho. I'm over the top, but not that much yet.' Today, with her wedding imminent and summer in full swing, the only dark cloud is, of course, the terrible news about Amy Winehouse, Ronson's long-term close collab-orator. Joséphine never got to know her. 'It's so sad, that's all I can say. Of course, she was invited to our wedding. It's a hugely sad loss.'

'De La Baume' means, in Old French, 'from the cavern' - 'Like a primate, no?' she growls. She grew up far from the caves of France, in Paris's Saint Germain des Prés, near the Ecole des Beaux Arts. As a teen, she carried the diary of feminist icon George Sand in her handbag, like a good young bohemian, and admired Simone Veil, who campaigned for women's reproductive rights. 'Growing up we lived in the middle of lots of galleries. It's fancier now but then Saint Germain des Prés was quite bohemian.' One of four siblings, she loves being part of a big family. 'I hope I can have lots of children. I'm going to keep my name for work, but also take Mark's name because I'd like to have the same last name as my children. I'm not super-conservative but a bit of tradition is nice. And "De La Baume Ronson" is quite a long name.' They are making a home in Notting Hill. 'Which is lovely and beautiful, but let's face it, it's not bohemian.' They have no sofa, as yet - 'It takes a while to make a home cosy' - but her prized possessions are there: a taxidermy bird ('They told me specifically it died from natural causes, but how can you know?'), a sheep made of pearls, from South Africa, lots of books (she recently loved Patti Smith's Just Kids) and an Elizabeth Peyton print of Frida Kahlo that Mark bought her.

Joséphine is 'a happy person, I'm not neurotic. I'm very comfortable with my body. I think it's great we're seeing more curvy girls in adverts now.' Her father is a financier turned theatre critic and producer; her mother works with disadvantaged young people (whose number once included Man on Wire's Philippe Petit). What did they make of her underwear campaign? 'They're my parents, of course they're going to judge me and say their piece, but at the end of the day they respect my choices and they're comfortable with what makes me happy.'

She read drama and photography at university; for nine years before that she trained as a ballet dancer: 'But I don't think I was made for that, with my curvy body. When I was a teenager, 18, 19, my body changed really quickly and it was a little confusing for me. Suddenly you can't eat Nutella every day. For a few years I was thinking about it too much. It's easy to become a little obsessional.' She does Pilates, boxing and cardio. 'I'm quite compulsive about exercise. For two months, I'll exercise every day, then for three months I'll do nothing. I love food, so exercise is important for me.'

Her film parts so far have included a lady-in-waiting in the historical epic La Princesse de Montpensier, directed by Bertrand Tavernier, and a 250-year-old vampire in Kiss of the Damned, alongside Milo Ventimiglia and Michael Rapaport. But perhaps her perfect role would be that of her grandmother, Suzanne Salmanowicz, should a biopic ever be made. 'My grandmother was very active in the Resistance,' she says. 'She lost her first husband when the boat they were in was torpedoed by a Spanish submarine and she was one of only five who survived. She was a spy who reported from concentration camps. She was really very brave. Meanwhile, it sounds silly, but she also had all her war uniforms remade so they fitted her better. She had them tailored by a designer friend. It's ironic because when she was torpedoed, she had her ass bitten by a shark in the sea off Africa, but because she had her coat remade in better fabric it saved her from getting too injured. It's a nice twist to the story: it's like her vanity saved her bottom!'

Joséphine has a peridot ring her grandmother gave her before she died, aged 90, a few years ago. 'It was given to her by the actor Stewart Granger; they were very good friends, apparently. She was super-glamorous, always perfectly presented, and really heroic. She did a lot for kids with disabilities and after the war she lived in England for a while.' This meant Joséphine grew up watching Fawlty Towers and Mr Bean - good preparation for life with an Englishman. 'I think the French have a romantic cliché that Englishmen have great style, great music, irony and sense of humour. Well, sometimes clichés are true.' And even before the wedding next month, one huge commitment has already been made: she points out where Ronson's initials are tattooed on her left ribs, in a tender place. ES
One Day is out on 26 August