Hannah Arterton steps out of sister Gemma's shadow with Walking on Sunshine starring role

Move over, the Cusacks, there's a new sister act in town. And between them they look to have the future of British musicals sewn up. This weekend Hannah Arterton makes her film debut in Walking on Sunshine, a sunny, 1980s-pop-soaked musical that is aiming to be this summer's Mamma Mia!. It will be followed in the autumn by Made in Dagenham, a big-budget West End show from the people behind Jerry Springer: The Opera and Enron, which will star Gemma Arterton.

At 25, Hannah is the younger sister by three years. Blonde hair aside, there's a clear family resemblance – the same snub nose and feline eyes, the same estuary vowels, throaty chuckle and girl-next-door charm. What's more striking is the spookily similar way in which both got their big break. Both went from the Miskin Theatre School in Dartford to Rada and both were plucked from final-year obscurity there by Stephen Poliakoff. Gemma appeared in his BBC drama Capturing Mary in 2007; four years later Hannah landed a part in My City at the Almeida Theatre.

Now that they both have leads in high-profile musicals, is it about to get a bit competitive between them? "No, no, no, not at all", says Arterton, fiddling with a tiny axe charm on her necklace. So far, her big sister has been nothing but supportive. When Hannah was 18 she lived in Gemma's London flat, auditioning for drama school and working in a Camden shoe-shop by day, cat-sitting by night as her sister roamed film sets and red carpets around the world. Sometimes, she got an invitation to join her. Her "work experience" the summer before she started at Rada was two weeks on the set of Prince of Persia, and she has had her share of plus-ones to award ceremonies and premieres. "Two girls who grew up on a council estate, sipping champagne next to Helen Mirren. Nutty," she says.

In other words, the Arterton dynamic is more Kylie and Dannii Minogue than it is Joan Fontaine and Olivia de Havilland. "She [Gemma] was so incredibly successful, very quickly. I might be making this up but I think she is the most successful female graduate of Rada ever", says Arterton. "She finished early, got a part in a film and then was just... off." A tough act to follow, then? "I thought: 'Oh god, I'm going to go to Rada and everyone is going to think I want to be on her coat-tails.' But we're so different. We have very different energies. I don't think we'd ever be up for the same part."

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In any case, if it's a hit on the Mamma Mia! scale, Walking on Sunshine has the potential to make Hannah Arterton a name in her own right. She stars as Taylor, who has a holiday romance with an Italian hunk only to return to the same beach three years later to find that her sister (Annabel Scholey) is now marrying him. Directed by Max and Dania, the duo behind the multi-million Street Dance franchise, it is a big, daft summer movie. Leona Lewis plays the best friend while Greg Wise takes the Pierce Brosnan, old-enough-to-know-better, role. There is an airport scene set to Madonna's "Holiday" (complete with hundreds of dancing baggage-handlers), a love scene on a sunset beach set to The Bangles' "Eternal Flame" and an argument with an ex set to The Human League's "Don't You Want Me". You get the feelgood picture.

For Arterton, getting the part was "a childhood dream" – and not just because she spent three months shooting on a beach in Puglia. She has been belting out the songs on the soundtrack for years thanks to The Hitmen and Her, a seven-piece covers band she has sung with at birthdays and weddings since college.

Growing up in Gravesend, Kent, she was always more interested in singing than acting. Not that it was all leotards and after-school clubs chez Arterton. "More like us doing routines to The Backstreet Boys and making our mum watch. It was unexpected that we decided to go into this weird acting thing. I think my parents thought, 'Oh, well it keeps them quiet...'" Her father, a metal worker, and her mother, who runs a cleaning business, separated when she was three years old. One day, when she was seven, her mother took her and her sister to the Miskin to sit in on a rehearsal of The Wizard of Oz, for which she had designed the set. "I'd never seen anything like that. I was absolutely blown away," recalls Arterton. "A whole different world just opened up." Later her sister's rise from Gravesend to Bond girl in a few short years, cemented her desire to perform. "I never thought I could really ever do it. But Gem's success... Someone from our background and our world achieving global success was just so inspiring for everyone. She had the talent, but more than that, she had the ambition. She didn't dream small."

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At 16, while she was studying performance and music technology and singing backing vocals for a skiffle band, she was offered a record deal. Then her A&R man vanished. "I thought: 'Argh, I can't do this. It's too hard to trust people in the music industry. It's full of sharks and guys latching on to a young artist and wanting to make money.' I found it all quite scary. " Her confidence knocked and unsure what to do next, other than sell trainers full time, she was persuaded by a teacher to apply for Rada. She auditioned, with a speech as Cleopatra, and got in. "I can't begin to describe how much I wasn't expecting that to happen. I wasn't even really taking it seriously. Well I was, but I wasn't thinking that if I didn't get in then that was my dream ended." The dream continued when she was cast in My City and found herself working with Poliakoff and Tracey Ullman, who, improbably, taught her to knit. "She's a champion knitter. We'd sit backstage, listening to Joni Mitchell and knitting."

After that, things became less idyllic. For 10 months, she could not get a job. "I think I had close to 100 auditions in that time and didn't land any of the jobs... I was exhausted all the time and I felt like I was still 17. I was earning the minimum wage in a bar, struggling to pay my rent, just trying to keep my head above water. There were so many times in that 10 months that I thought: 'I'm just not very good, I'm not supposed to do this. I should, you know, think of a back-up plan.'"

She has no idea what that plan might have been, but was never tempted to play on her famous surname. "No, absolutely not. I don't want to get any kind of job unless I'm really right for the part. I think a lot of actors leave drama school and think that if they're not in a movie within the first year that they're failing. But that isn't true. Some of the best actors I know go through that time of real struggle. It just makes you better at your job."

She was considering going back to the shoe shop when she landed a role in the big-budget BBC fantasy series Atlantis, swiftly followed by Walking on Sunshine. Another British film, Hide and Seek, premiered at Edinburgh film festival last week. She plays one of four young, frequently unclothed, adults who leave London to set up a free-thinking utopia.

She lives in London with her two best friends from Rada, Sophie Rundle (Peaky Blinders) and Eline Powell. Her boyfriend, Andrew Hawley, is also an actor. "But I don't want to talk about him too much." She'd rather talk about Mouse, a two-month old sausage dog she takes for long walks in the woods whenever she can. "I think it's important to detach and get out of the city and the acting world, the industry mess... "

She may find that more difficult to do in future. "The only thing that I have learned through seeing Gem do it is that I definitely don't want to be famous. It's odd, because you are just a normal person but people are so knocked out to see you. I don't want to not be able to walk down the street. I don't want to have to wear a wig to Tesco."

"There was a point a few years ago where it was a bit crazy but my family is so great. We go round to my dad's for a Sunday dinner and he'll say: 'Right, we cooked so you girls are washing up'. We don't get away with anything."

'Walking on Sunshine' is out on 27 June