Modern miss: Hattie Morahan is ditching bonnets in favour of cutting-edge theatre work

When Hattie Morahan played the dowdy, sensible Elinor Dashwood in Andrew Davies' adaptation of Sense and Sensibility, the television critics lavished her with fulsome praise worthy of a love letter penned by Mr Ferrars himself. Her "winningly unshowy performance" was described as both "luminous" and "exceptional". "As good a piece of acting as you're going to see this year" declared one review – a particularly bold claim given that it had screened on 1 January. Even Davies, who had apparently objected to her casting, declared that he had "fallen in love with her performance".

You won't catch Morahan indulging in such romantics, though. Her approach to the role – immortalised for many by Emma Thompson's Oscar-nominated performance in the 1995 film – was resolutely pragmatic. "You are aware of the precedents but you just try to shed all that. You have to keep thinking, 'what would my character be thinking about?' It's not, 'Ooh, I'm in a bonnet and I look terribly 1800s.' They're thinking, 'I'm going to put my hat on now because it's cold,'" says the 29-year-old actress. "We wanted to make it feel as natural as possible. A little mental alarm bell would go off if we found ourselves sitting in a very formal position. We'd say, 'we're getting a bit costume drama here...'"

And so, unlike Jennifer Ehle and Colin Firth, whose faces will forever be associated with their leading roles in Pride and Prejudice, for now at least Morahan is defying the pigeonhole of classic English rose. In the last year she has also appeared in the Hollywood blockbuster The Golden Compass as the brisk Sister Clara, who severs children from their daemons, as a sexy, 1970s MI5 agent in the Brit heist flick The Bank Job, and, most recently, a neurotic wife in Martin Crimp's creepy dystopian drama The City, at the Royal Court.

Today at the National Theatre Studio she is unrecognisable as the actress who played the pale and uninteresting brunette Dashwood. She's dressed in jeans and baseball boots, her striking green eyes peering out from underneath a messy pile of blond hair, and her voice, husky from a recent virus, is a far cry from the clipped formality of Austen-speak. "I've obviously got one of those faces that people can forget," she says, shrugging off the suggestion that her period drama success has earned her legions of fans. "Throughout my life I've known that if I change my hair, if I change my look, people I know will blank me in the street."

Her next incarnation will be as the mesmerising beauty with a dark past, Nastasya Filippovna, in a new version of The Idiot. Directed by Katie Mitchell and starring Ben Whishaw, ...some trace of her will take the same impressionistic approach as Mitchell's Waves did to Virginia Woolf's novel in 2006, incorporating live video into a fragmented stage narrative. "There are a million different ways you can adapt something. If you were making a 10-part series with a huge budget, filmed on location in Russia, you could service all the different elements," says Morahan. "To start with you think you need a five-page scene to get something but then you realise it can be told in one shot, a look and a sound effect."

This is, famously, the modus operandi of Mitchell, a theatre director who has made her name with audacious rewrites of literary behemoths, in this case shrinking Dostoevsky's hefty novel down to a 60-page script. "There have been times when we've said: 'Gosh, this is a really well known bit of the book, can we really cut this out?'" admits Morahan. But she must be used to the process by now, having previously taken the title role in Mitchell's stunning Iphigenia at Aulis and playing Nina to Whishaw's Konstantin in The Seagull. Though that production split the critics – one notoriously branding it "bird-brained" – Morahan won an Ian Charleson Award for her desperately vulnerable performance. Does she worry about working with a director who attracts praise and brickbats in unpredictable measures? "Gosh, I mean, one is aware that her work divides people," says Morahan. "But I'd prefer that to an experience where the director is just trying to please as many people as they can."

With Nastasya, who comes hot on the heels of The City's overwrought Clair, Mitchell has once again cast Morahan as an "emotionally volatile" female. But while she has a certain wide-eyed fragility, in person the actress is down to earth. It's an attitude, most likely, born of a life spent in the theatre. The daughter of the actress Anna Carteret and Jewel in the Crown director Christopher Morahan, she recalls a childhood spent on film sets, at Laurence Olivier's parties and on Broadway. Her half-sister translates for the theatre and her two older half-brothers work as a commercials director and a production designer. At school she briefly flirted with the idea of becoming an artist – but never seriously. "I've often thought, 'God, if this all fell through, what would I do?'"

In typical teenage fashion, it was not her parents' example but Stephen Dillane's Hamlet that inspired her to tread the boards. She got an agent, aged 16, when her father cast her in The Peacock Spring, a television drama in which she played a young 1950s girl who falls in love with an Indian boy (Lost's Naveen Andrews). But her parents were "never pushy, quite the opposite. They wanted it to be pretty clear that it wasn't an easy ride."

After studying English literature at Cambridge – where she auditioned for 11 plays in Freshers' week – she decided against drama school, to the dismay of her family. "I'd been in education for ages and you're so restless at that age..." The gamble paid off: she was offered a contract by the RSC and did her training on the job instead. These days, she's more open to parental advice. "My mum will talk to me about playing a part, or I'll ask for notes after they've seen me. It's symbiotic, fruitful."

While at university, she met her fiancé, Blake Ritson, the actor who played Edmund Bertram opposite Billie Piper in Mansfield Park. With his older brother, Dylan, a writer, he has just finished his third short film Good Boy, starring Jessica Hynes, Gavin and Stacey's Joanna Page and Morahan – who also helped out with scripts and costumes. "I know some actors are wary of going out with other actors but it's lovely because you can share so much," says Morahan. "Any job that's vocational takes up your imagination and you bring to it more than just: 'this is my job and at 5pm, I leave.' Both of our jobs are who we are."

She still feels most "at home" on the stage. "There's a lot more paraphernalia to get used to on a film set. I suppose it tends to be more commercial and I've found it less consistently rewarding," she says. "Sometimes it's banal things like having to get up really early. Acting at 8am? You're thinking, 'My body hasn't woken up imaginatively at all!'"

That said, her experience on The Golden Compass was "really good fun – kids, animals, all the things they say you shouldn't do. It was fascinating to see something of that scale. You're aware that you're one tiny cog in a huge machine." She widens her eyes in horror. "The pressure of being 'the name' in something like that must be phenomenal." Something tells me Hattie Morahan might just have to get used to the idea.

'...some trace of her', National Theatre, London SE1 (020-7452 3000), in rep to 21 October