Our new favourite mademoiselle

The BBC's recent adaptation of Birdsong, Sebastian Faulks's much-loved novel of love and tunnelling in First World War France, may have split the critics, but it did concentrate attention once again on the beautiful, expressive features of Clémence Poésy. The 29-year-old Parisian actress played Isabelle, the unhappily married industrialist's wife and the object of Eddie Redmayne's rapt attention (note to Eddie; close your mouth when staring at a lady), and Poésy is fast becoming the go-to French actress for English-language TV dramas and films.

From mainstream witch to indie darling – or, more precisely, from Fleur Delacour in the Harry Potter series to Chloe in In Bruges – Poésy is increasingly bestriding the Anglo-Gallic film-making divide, a duality neatly encapsulated last year when she played both the French national icon, Joan of Arc, in the Cannes-competing Jeanne Captive, and a piece of arm candy in the hip US cable show Gossip Girl.

"I feel very privileged to have been welcomed in England in that way," says Poésy..Her latest English project has also been her most daunting – and although the role of Queen Isabella in Shakespeare's Richard II is only a minor one, the challenge of mastering iambic pentameters for the BBC's upcoming cycles of Shakespeare plays was considerable.

"It was like learning how to speak another language," she says in accented but perfectly fluent English. "You do Shakespeare at drama school but you do it in French. It's interesting to see, when you study theatre in France, the different translations of Shakespeare – because obviously in England you just work on one material."

She learned from her Bard-hardened Richard II co-stars, Rory Kinnear, Ben Whishaw and Patrick Stewart, and reflects with amusement on how she got into the Conservatoire National Supérieur d'Art Dramatique (France's equivalent of Rada) by performing, in English, Juliet's balcony scene from Romeo and Juliet. "No one had any idea of whether it was any good," laughs Poésy, who took her mother's maiden name for the stage. Her father, Etienne Guichard, is a theatre director, who used to pretend to Clémence and her younger sister, Maëlle, that their TV only played videos of movies. After a stab at couture that ended after a disastrous work-experience placement, Poésy grudgingly accepted her thespian fate.

"I was the one in the family who was saying I wanted to do something else," she says. "Mostly because I felt a bit silly saying that I wanted to be an actress before I actually was an actress – or it might have been being scared of failure."

A string of French roles playing teenagers ensued, before her English-language breakthrough as Mary, Queen of Scots in Jimmy McGovern's 2004 BBC drama Gunpowder, Treason & Plot – a role that led to Harry Potter. Her mother, a schoolteacher, had already encouraged her to read JK Rowling's books.

Harry Potter led to a variety of English language parts, from the aforementioned In Bruges, with Colin Farrell ("people love that film"), and the 2007 TV mini-series War and Peace, to playing Jim Sturgess's enigmatic girlfriend in the London-set horror film Heartless, and as James Franco's lover in Danny Boyle's 127 Hours. Now Poésy is involved in a somewhat more unusual romance, Mr Morgan's Last Love, an age-gap meeting of lonely hearts between a free-spirited Parisian and Michael Caine's retired and widowed philosophy professor. It sounds like Lost in Translation.

"Yuh, it's two lonely people finding each other, except it's Paris and not Tokyo," she says. "It's not a real love story but there's a lot of love in it... It was lovely to get to know him. He's incredibly simple, and he's got a very playful approach to the whole thing still. "

Poésy is also a musician (she plays guitar, and sang on last year's debut album by the Last Shadow Puppets' Miles Kane) and fashion icon – a face of the perfume Chloé and now the new face of Dutch urban fashion chain G-Star Raw .One poster we won't be seeing however, is of a naked, or semi-naked, Poésy. After a bad experience as an 18-year-old starlet, she has a clause in all her contracts that states any nude scenes she films can't be used in trailers or publicity stills. "People can find the scene and so whatever they want on the internet," she says, "but at least they can't use it on the trailer."

Birdsong is out on DVD on 12 March. Richard II is on BBC2 in July. Mr Morgan's Last Love is released this autumn