If it’s not Clémence Poésy's effortlessly chic outfits we’re lusting after, it’s her choice of co-stars that has us green with envy. After sharing screen-time with James Franco in 127 Hours and Colin Farrell in In Bruges, she’s now starring opposite Orange BAFTA rising star nominee Eddie Redmayne in the BBC’s adaptation of Sebastian Faulks’ First World War novel Birdsong. Stylist grilled the French actress on fashion, fiction and working with one of this year’s most exciting men.

Birdsong is beloved by so many people, did you feel daunted taking on a piece of work that people feel so protective of?

The book isn’t as big in France as it is in England, but the minute people started telling me how much they loved it, I got nervous. A lot of girls talk about the novel in a really passionate way and I think that’s a lot to do with the fact that it’s set during the war, but also with how modern Isabelle’s character is for a woman in a period novel.

She’s quite a complex character, how did you approach the part?

I find that there are two types of parts in acting. There are the clear ones that you can just dive into and there are more opaque characters that you have to circle around for a long time before you understand why they do what they do. I know my Isabelle isn’t going to be the same as everyone else’s, but you have to give that up at some point. For inspiration I listened to a lot of Cat Power; her songs are sad but at the same time there’s something very independent and strong about her.

It’s quite a hard-hitting book. Did you find any of the scenes emotionally draining?

As an actor it’s part of the job to play with your emotions, but I was actually surprised by some of the scenes. There were certain points that became really intense when we were shooting them. Like one scene where Stephen and Isabelle touch ankles for the first time on a rowing boat; while we were shooting it I just couldn’t stop crying. It was really weird. I had no idea it was going to go there.

Eddie Redmayne is your co-star in the drama. What was it like working with him?

He is just the easiest, gentlest, most generous and open guy. He’s got this amazing academic education that he mixes with an incredible freedom, so he’s got the best of both worlds. It’s weird how a leading actor, just by his way of being, can cast a mood on set; when he arrives his energy just touches everyone. A lot of the crew loved doing Birdsong – and it was a hard job because they were filming in the trenches – and that was a lot to do with Eddie.

The novel is famous for its sexual content, was it hard filming some of the more racy scenes?

We had three days of rehearsals in London and talked those scenes over and over. I could really confide in Eddie and we were both very lucky because we were totally at ease with Philip Martin the director, which is a huge deal because if you’re not, those scenes can become a nightmare. If viewers knew what actors go through when shooting scenes like that, they’d never find them sexy; it’s all very technical. But we were very conscious that we had to make them work. I normally try to escape nudity but actually, for the first time in my life, we added more sex scenes. We really wanted to get it right.

So, tell us about the costumes...

Oh my gosh the costumes are amazing. I think Isabelle lives in paintings. There’s a sentence in the book that describes her as being not from her time but from somewhere in her imagination. She is trying to make her life beautiful, because it’s not, until Stephen turns up. Charlotte, the costume director, had such wonderful attention to detail. She gave me this one dress that she’d had since the 70s and it was completely falling apart. We knew it was its last run and it’s so nice when you’re trusted with a costume like that.

Did you have a favourite outfit?

It was that dress. It was blue, but a special type of blue you would never be able to get if it hadn’t gone through all those years. And then there was a bright, almost fluorescent green dress that I loved. I’m almost surprised when I do a period film because we always see black and white pictures, so when you see those costumes and you realise they were living in colour, it’s striking really.

You’re a familiar face on best-dressed lists; how would you describe your style?

I always feel really silly answering that question because I really don’t feel like I have a style. I like it when clothes have a story and I like finding things that move me somehow but I think it’s very important to not make fashion too much of a thing in your life. It’s a tool to show who you are to the world but only if you’re privileged enough to enough be able to worry about it. It can be armour, it can be something you protect yourself from the world with, it can be something that gives you power that makes you feel stronger but it’s important to see the poetry in it and try to tell a story with your clothes. Not to make fashion too much of a thing.

Clémence reveals her favourite shopping destinations here...

Words by Lizzie Pook

Birdsong starts 9pm, Sunday, BBC One