Currently offering an enigmatic smile on the cover of Vanity Fair’s annual Hollywood issue, you have to wonder how Carey Mulligan managed to stop herself beaming wildly when Annie Leibovitz took that snap. Snagging Golden Globe, Bafta and Oscar nominations for her role in An Education, in which she beautifully essayed a 16-year-old girl who falls under the spell of an older man, it’s staggering to think that she was an unknown just twelve months ago. That was before she was tagged as a ‘one to watch’ in European Film Promotion’s Shooting Star actor showcase, and before An Education and The Greatest premiered at the 2009 Sundance Film Festival. Shortly after these events, Mulligan – still only 24, lest we forget – was hailed as the year’s most exciting discovery.

For once, it’s not proved to be hyperbole. While she’s proved herself adept in shouldering a lead in An Education, she’s been just as potent in supporting roles – notably as the wife of a missing marine in Jim Sheridan’s recent Brothers. Much the same can be said for The Greatest, which receives its UK premiere at the Glasgow Film Festival. US trade paper Variety praised her ‘bracing resilience’ as Rose, a young pregnant women whose 18-year-old boyfriend is killed in a car crash. With Pierce Brosnan and Susan Sarandon cast as his parents, the film ‘lasts the duration of my pregnancy’, as Mulligan puts it, as they take Rose into their home and prepare for the baby’s birth.

Despite her appearance in Brothers and also playing a prostitute in Michael Mann’s John Dillinger biopic Public Enemies, the London-born Mulligan estimates The Greatest is ‘my biggest American accent role’ to date. Given she will soon be seen in Oliver Stone’s Wall Street sequel Money Never Sleeps, playing daughter to corporate raider Gordon Gekko, that may change. Yet The Greatest was something of a baptism of fire for the elfin-like Mulligan, who admits she felt ‘inhibition’ speaking with a Stateside brogue. ‘It’s fear of sounding silly in front of other people – especially Americans. You don’t want to have a dodgy accent and let people down.’

While the film deals with a family coming to terms with grief, Mulligan maintains it was still a happy set to be on. ‘It had to be otherwise it would’ve just been horrible experience,’ she says, ‘being miserable all the time.’ Even so, she found playing a supporting role was no cakewalk. ‘You come in and have to sense the tone of the piece and see if you fit in. And you don’t know anybody, and you’re nervous and you’re inhibited by not knowing people. When you’re the lead, you come in every day. The runner becomes your best friend because he’s the person who gives you coffee in the morning. So you can do whatever you like. You never feel uncomfortable, you never feel you’re making mistakes because these people are trying to make this film with you.’

In truth, Mulligan’s days of playing support are probably numbered. Having just completed a role in Rowan Joffe’s remake of the classic tale of seaside violence, Brighton Rock, after her Wall Street sequel, she will be seen in Mark Romanek’s adaptation of Kazuo Ishiguro’s novel Never Let Me Go. It reunites her with Keira Knightley, whom she met on her first film, Joe Wright’s 2005 take on Pride & Prejudice. Does she think Knightley sets a good example to follow? ‘Yeah, she does,’ she nods. ‘She has a lot of bad stuff to deal with. She gets a really hard time from the press sometimes, and she deals with it really gracefully. Also, she’s one of my best friends, so it’s hard to talk about her really.’

Expressing herself with a quiet but steely confidence, Mulligan – whose mother is a college lecturer and father a retired manager for Intercontinental hotels – admits it wasn’t always this way. ‘I was just quite shy,’ she says. ‘I didn’t really have a proper relationship until I was 19. I was kind of a loser.’ Then she did Pride & Prejudice and something shifted. ‘I fell in love with everybody on that film. I was in love with [co-star] Rupert Friend for like a month. He’s one of my good friends now. It was like, “Oh, boys!” So I was like “You, you, you and you” – the boom, the sound guy, lighting, yes, everyone. I’d not really been around! I didn’t end up with any of them. I had very romantic crushes.’

Now dating her Wall Street co-star Shia LaBeouf, the good news is the angst-ridden dreams have also stopped for Mulligan. A year ago, just before Sundance, she started ‘freaking out’ that she was going to be lousy in An Education and The Greatest. ‘I had these anxiety dreams – very basic anxiety dreams,’ she says, giggling at how ridiculous this seems now. ‘Geoff Gilmore, the [former] director of Sundance put me in a car and made me leave for being disappointing. In Park City, in the main high street, he gets this black car and he’s like, “Get in. You’ve got to go.”’ What a difference a year makes.

The Greatest, Cineworld, Thu 25 Feb, 9pm & Fri 26 Feb, 1.30pm.