Alexandra Roach will never forget the first time she walked on to a film set. It was her first day at work on The Iron Lady, and she was about to film her first scene as the young Margaret Thatcher and, indirectly, the young Meryl Streep. "And Meryl Streep came on set to watch me. Stood behind the camera. Just casual, like," she says, in her Swansea sing-song. "I really had to have a word with myself. 'What would Maggie do?' After my first take I looked over, to see if she was still there and she just gave me a wink and a thumbs-up."
"Once I met her, she didn't intimidate me at all," she adds. "She comes straight down to your level." Dawn calls to the make-up trailer are another great leveller, of course, and the two actresses spent countless hours together having their false noses stuck on in preparation for playing the British Prime Minister from her teenage years to her dotage. "We didn't share a trailer. She's got her own city… But when she was being made up as old Maggie she would be in prosthetics for hours and I'd just go in and have my nose put on, my teeth fitted. It was a bit random, lying there next to Meryl. But she talked to me about normal things."
As handles go, the young Meryl Streep is not a bad one but Roach, 25, is well on the way to making a name for herself in her own right. Indeed, she's in danger of becoming ubiquitous. In the past couple of months she has played the shallow society snob Countess Nordston in Joe Wright's Anna Karenina and taken the romantic lead in Hunderby, Julia Davis' twisted Rebecca spoof for Sky Atlantic. She is currently back on the big screen in Private Peaceful, a sweet adaptation of Michael Morpurgo's wartime weepy – think War Horse, without the horses. "Or Spielberg's money," says Roach, drily.
For now, thanks to that fake nose, she is still able to walk down the street unrecognised. "I think I've got one of those faces which looks different on screen than in real life," she says. "So I'm told by cameramen, anyway." Today, dressed in a coral jacket, jeans and trainers she is pretty but normal-looking. Put her in front of a lens, though, and her pool-like eyes, creamy skin and dark hair morph into something quite striking.
It's this ability to transform on camera that has already made her a go-to for playing real people. Her first lead part was Constance Kent in the television adaptation of Kate Summerscale's The Suspicions of Mr Whicher, itself based on a real Victorian murder case. Next came The Iron Lady in which she played Margaret Roberts/Thatcher from the age of 16 to 33. In the new year she will dust off her RP to play the young Queen in Girls' Night Out, a historical drama set on VE night. "The two princesses were let out of the palace for one night, to celebrate with the people. No one knows what happened that night so this is the rom-com version. They go to the Ritz, run away from their minders and have a midnight adventure with a Cockney boy," she says. On a different note, she has just joined the cast for One Chance, a new biopic of Paul Potts, the Carphone Warehouse salesman turned Britain's Got Talent winner and multimillionaire opera star, directed by David Frankel (The Devil Wears Prada). She will play the wife to James Corden's Potts.
Forget the young Meryl Streep, she's the new Michael Sheen – "Yeah! I'll run with that" – already rivalling the actor's tally of real-life characters and still only two years out of Rada. She grew up in Ammanford, half an hour from Port Talbot, the town which famously spawned Sheen, Anthony Hopkins and Richard Burton, among others. Is there something in the water there? "Every time I go through Port Talbot I do wind the window down and breathe in all of the pollution that comes from the steel works," she says. "Just in case."
Other than that, there's nothing theatrical in Roach's background. Her mother is a PA to the Children's Commissioner for Wales and her father, a former policeman, now works for Welsh Rugby Union in regional development. Her brother and sister are both in the police force. She goes home as often as possible to watch rugby and walk the dogs. "That's when I'm really happy and at peace with everything," she says.
Her breakout role, though, could have changed all that. Ammanford is a tiny town with a population of 5,000 and, once upon a time, three coal mines. Her grandfather was a miner from the age of 12 and her father was a policeman during the strikes, which made for an interesting household. When Roach landed the part, it stirred up old tensions. "My Dad said, 'We'll never escape Thatcher. Not with you playing her...'" she says. "I went back to the social club at Christmas and I was a bit nervous about it but there was no animosity at all."
Nevertheless, the film provoked vociferous reactions – simultaneously criticised as a soft-soap and an attack on an old woman. "I knew that it was going to be controversial from the moment I read the script," says Roach. "It shows her in such a vulnerable way that did make a lot of people uncomfortable. I don't think that's a bad thing, to shake things up. My role was to understand her and not judge her."
Roach has been acting now for more than half of her life. "Fourteen years! It's men-tal," she marvels. At school she had little interest in drama, always playing "third angel from the right or a comedy old woman". One Saturday, with nothing else to do, she tagged along to the local drama club with a neighbour. By chance, the long-running Welsh-language soap opera Pobol y Cwm was holding auditions and Roach was snapped up. From the age of 11 to 18, she played local troublemaker Elin Owen, "smoking drugs, stealing my mum's credit card, fancying the vicar… that sort of thing".
She applied to Rada straight from school and got in on her second attempt. It was, she says, a stressful time. "I didn't find it a very creative environment at all." In fact, it made her ill. In her first year, she suffered a back injury and had to sit out the rest of the year, depressed, at home, doing physio with her Dad's rugby team to recover. Back at Rada, she was cast as Juliet but a difficult director set her back once more. "I got really stressed with it and started fitting. I'd have four fits a day and ended up in hospital." It turned out that she suffers from Non Epileptic Attack Disorder. "My brain is like a computer, if I have one overriding emotion then my brain freezes, and to reset it, it needs to fit," she explains. "It's not at all common. Apparently Julius Caesar had it."
Still, two good things at least came out of Rada. The only Welsh girl in her year, she didn't fit in with the Eton and Oxbridge types and fell for her (similarly heavily accented) Glaswegian classmate, James McArdle. They now live together in Muswell Hill, north London. He is currently starring in Chariots of Fire in the West End. "I've seen it six times now," says Roach. "It's the only time we get to spend together."
The other good thing was that she was spotted by a casting director and invited to audition for The Iron Lady. Having "messed up" her first go, she gave her everything to the callback. She bought a blue suit from a charity shop, borrowed a vintage handbag from her drama teacher and had her hair curled, 1940s style. "And I listened to classical music on the bus on the way there, to make me feel really grand," she recalls. "There was a girl next to me at the audition in skinny jeans and Converse so I felt a bit of a prat but I stuck with it." A couple of days later, she heard that her tape had been sent to Streep for approval. "She said: 'She brought humour to the part and any great actress brings humour to a part,'" she says shyly.
She would, she says, much rather do a comedy than a period drama. Hunderby, which she fondly refers to as "filth in a bonnet", happily combines both. "I didn't get through one scene without laughing. Some days it was impossible. I had to think to myself that it was my Jane Eyre, my Tess of the D'Urbervilles." She has also just shot Cuban Fury, Nick Frost's new comedy with Chris O'Dowd and Rashida Jones and is currently filming Utopia, a new six-part conspiracy thriller for Channel 4 from Dennis Kelly (Matilda) and the makers of Spooks.
As for the future, she would love a holiday. "I've been on the go for two years," she says. "I need to go and plug into a wall somewhere and just sleep." Given that seems unlikely on her current trajectory, she'd also love to play Juliet. "I need to do some theatre before I get scared of it. I've lost all confidence with Shakespeare after Rada. It's one of the demons that live inside me. I need to exorcise that and do Juliet again before I get too old. Like my Dad would say, 'Tackle it. Get it out.'" And she already has her eye on another real character, her heroine, Marilyn Monroe. She has a poster of her above her bed. "So she's the first thing I see in the morning. Has there been a full biopic yet? I'd be happy to take that on."
'Private Peaceful' is out now