Blonde ambition: The unstoppable Sheridan Smith

Sheridan Smith triumphed in Legally Blonde. Now she's starring in a Rattigan revival, likely to play Bridget Jones on stage – and favourite for an Olivier Award this weekend

Being a West End leading lady can take a little getting used to. After a triumphant 14-month run as the hot pink star of Legally Blonde, garlanded with five-star reviews, nightly standing ovations, a Whatsonstage Award and, most likely after this weekend, an Olivier, you'd think Sheridan Smith might have got used to it by now. She hasn't. She's late for our interview because she couldn't find the stage door of the Theatre Royal Haymarket – it's the opening week of her latest play Flare Path and the company's first day in the theatre – and while she was looking for it, she bumped into two "little old ladies" from her village in Lincolnshire and had to stop and chat.

"This is nice," she says, settling into an armchair in her new dressing room. She nods approvingly at the giant mirror and a costume rail hung with pretty 1940s dresses. "I had my own shower at the Savoy but still... very nice." A few minutes later, her co-star Sienna Miller bursts in to say hello. "How lush is this?" squeals Smith, crushing her in a bear hug. "Amazing! Look at my gold plaque!" She puts on a faux posh voice. "Miss Sheridan Smythe...!"

As the door closes behind Miller, Smith leans forward and whispers, reverentially, "See? She's lovely, isn't she?" A little later, the stage manager pops his head in to offer her lunch. "No, I'm alright babes," she says, gesturing at her handbag. "I've got a pasty and some Minstrels in here." Later, she excitedly posts a picture of her dressing-room fridge, stocked with opening night champagne, for her 136,777 followers on Twitter. She's obsessed with Twitter, replying to all of her fans even though her "people" tell her not to, shouting out to them in awards acceptance speeches and dispensing advice to her "Twitfam" ("I've got 14 adopted daughters now and one adopted son"). When one of her dogs, a Shar-Pei called Enid Blyton – she has two more, a Japanese Spitz called Patricia Passion and a black pug called (what else?) Barry Manilow – was stolen from her back garden last year, she mobilised her "Twitter Army" to get her back. "I'm always Tweeting them. I'm not just going to use it to say I'm on telly on such-and-such a date. I want to know them. I never feel that there needs to be such distance. I'm just some commoner trying to work in acting."

In case you hadn't realised, you don't get hauteur or icy, don't-you-know-who-I-am celebrity from Smith. But don't be fooled. This "little knobhead from up North" (her words) is currently one of Theatreland's most powerful. In January, the 29-year old was the only actress (and one of only two actors: Mark Rylance was the other) to make the top 20 of The Stage's guide to the 100 most influential people in British theatre. A tiny blonde bombshell in studded ankle boots, Smith is now that rare beast, a female with the power to open a West End show on her name – and lungs – alone. These days, she gets texts from Trevor Nunn begging her to appear in his plays and already has her eye on her next big role, playing Bridget Jones in a new musical, with songs by Lily Allen. On both of which – more below.

"I always think, well, I can't do anything else," shrugs Smith. "And I don't even know if I'm that good at this." She still doesn't think of herself as a "proper" musical theatre actress. "I'm never going to be able to play in Phantom or anything like that." She hates posing for photographs, finds interviews awkward and thinks fame is "quite mad, really". The idea of someone being starstruck by her baffles Smith, though she gets starstruck all the time by people "off the telly"; even, sometimes, by people off the adverts. "I don't know why, it's just a job," she says. "I certainly never get above my station – my family would soon slap me back down to earth."

In some theatrical types, this relentless normality could be just another affectation; but there's a vulnerability to Smith that makes you think it's probably not an act. The other day she had to go on The One Show and was, she says, dumbstruck with nerves. "People are so blasé about it. But when you're hiding behind a character all the time, as soon as you have to be yourself, you feel kind of terrified." A professed "trackies-and-trainers girl", today she looks more glamorous than grungey in giant hoop earrings and skinny jeans, nails painted to match the leopard-print bra peeking out of a beige silk shirt. She's only recently started to enjoy dressing up for posh parties, having been burned by the odd lairy paparazzi shot. "Maybe I'm going from that tomboy-ish state to feeling a bit more womanly. I've enjoyed wearing some fancy frocks. It's nice once in a while." When she goes to the Olivier Awards on Sunday night, a shoo-in to win Best Actress in a Musical for Legally Blonde, she'll have her father on her arm. "I can't wait to walk up the red carpet with him. You don't do it for awards, of course. But I doubt myself a lot. To get them is a little pat on the back that you're doing something alright, that you're on the right path."

Before that, she has another opening night to tackle. She's starring in Flare Path, one of a rash of Rattigans hitting theatres for the playwright's centenary. Based on Rattigan's own Second World War experiences as a tail gunner in the Bomber Command (the first draft was the only thing he saved when his plane came under attack, apparently), the play is set over one long night in an airfield hotel and features a love triangle between Patricia (Miller), her pilot husband, Teddy (Harry Hadden-Paton), and Peter, a dashing film-star (James Purefoy). Smith plays Doris, a hotel guest and chirpy, dipsomaniac barmaid who has married a Polish Count. "She's quite common, really," says Smith. "It couldn't be more different from an LA rich girl with a Chihuahua in her handbag."

The actress hadn't planned another West End jaunt so soon. Having extended her Legally Blonde run by a couple of months to allow fans who'd been given tickets for Christmas a chance to see her, she finally hung up her pink stilettos on 8 January. She then jetted off to Dubai – a late Christmas present to her family and her first time off in nearly two years. "We hadn't had a family holiday since I was six when we went on a bus trip to Spain," she says. "It was great but two days in, I got the text." The text in question was from Nunn, asking if she wanted the part of Doris – no audition required. She thought he'd sent it to the wrong person at first but then they sent the script out to Dubai for her to read by the pool. "I cried, got some funny looks and thought, 'Do you know what? I can holiday another time.' How can you say no to that?"

She is learning to say no. "I have options, which I didn't have before," she says, modestly. In fact, her mischievous turn in Legally Blonde as Elle Woods, the ditzy LA fashion student who follows her boyfriend to Harvard Law School, propelled her into a different league. The critics fell over themselves to praise Smith's "brilliantly warm, winning, witty and all-round adorable performance" (The Independent), her "star quality" (The Guardian) and her "vitality, warmth and great comic timing" (The Daily Telegraph). In other words, she got the kind of star-is-born notices most actresses only enjoy once in a lifetime.

Smith enjoyed them once before. Having made her West End debut aged 16 as Tallulah in a National Youth Music Theatre production of Bugsy Malone, she was picked up by an agent and starred in Into the Woods at the Donmar a year later, where she was tipped for stardom. The Royle Family followed, playing Ralf Little's girlfriend. For the next decade she pursued a path in sitcoms and comedy dramas, a professional tart-with-a-heart sidekick. They ranged from the good – Smithy's loudmouth sister in Gavin and Stacey, Jonathan Creek's assistant – to the bad (Two Pints of Lager and a Packet of Crisps) and the occasionally ugly (Benidorm). "Chavs and slappers are a speciality," she admits. "But I try to do different things with them all." No need for drama school, she learned on the job, soaking up the skills of her venerable comic co-stars – Caroline Aherne, Sue Johnston, Caroline Quentin – "like a sponge".

Eight years later that she found her way back to musicals in Little Shop of Horrors, which transferred from the Menier Chocolate Factory to the West End and earned her a first Olivier nomination. After she performed a song from the show at the 2008 awards, the director Jerry Mitchell, fresh from opening Legally Blonde on Broadway, came up to her in the bar and said, "You're going to be my Elle Woods." Unsure if he was drunk or joking, Smith nevertheless set her sights on the role. "I said to my agent, 'You need to get me seen for this part.'" She wasn't an obvious choice – apparently Mitchell had to teach her how to cross her legs "all girly" – but her tongue-in-cheek take was a winner with British audiences. She gave up smoking and drinking and threw herself into the gruelling routine – 18 numbers and 16 costume changes a night. "It was," she says, beaming. "The best 14 months of my life."

She's now eyeing her next role, in Bridget Jones' Diary, set to be brought to the stage next year by Working Title. Smith recently did a workshop with Lily Allen, Helen Fielding and Stephen Daldry, who directed both film and stage versions of Billy Elliot, and would be perfect casting as the perennially single girl next door. "Every girl at some point feels like Bridget Jones. I'm constantly wearing Spanx, eating ice cream, and feeling a bit lonely. Parts like Bridget and Elle are the parts I like – real girl power. Bridget is great because she's more downtrodden. She falls over all the time, or makes an arse of herself and I'm totally like that. I always joke that I'm going to be a spinster with 100 dogs, live on a hill and smell of wee," Smith cackles. "I'm 30 this year and you have those moments."

Is she single? "Very!" Smith split up with her Gavin and Stacey co-star James Corden before Legally Blonde began. "I watched him on the Brits and I was beaming with pride," she says. "When you've been together and had happy times, all you want is for that person to be happy." Since then, she hasn't had time for a boyfriend. "I didn't want to lose focus. That was one of the biggest jobs of my career. I didn't want to get distracted with boys. I thought once I finished Legally Blonde, I'd have my evenings free, maybe I could go on a few dates. And now I'm back in the theatre. So it's a lonely old life." She wouldn't swap it, though. "Every job I've done, I've thought, 'I'll move back home soon, back to my burger van.' I feel so thankful that I don't want anything to get in the way of it," she says. "I love my job. It's quite selfish, I suppose, but that's how I feel at the moment."

Performing is in her genes after all. Born in Epworth, a village between Doncaster and Scunthorpe, she's a "country bumpkin" with country music in her blood. Her parents had a country-and-western duo called The Daltons and gigged seven nights a week, in pubs and working men's clubs. Smith would go along and perform, standing on a stool. "Kids' songs like 'Grandpa' or 'They Don't Make 'Em Like My Daddy Any More'. There's nothing worse than seeing a kid singing love songs, it's so precocious." She still sings with them at the Top Club in Bentley – these days, it's Patsy Cline's "Crazy" – and plans to celebrate her 30th at Dollywood, the home of Dolly Parton, "in a pink cowboy hat, being very camp".

They weren't pushy parents: Smith pushed herself and, by the age of 16, was living in London (with, fact fans, Hannah Spearritt from S Club 7) and earning a wage. When her brother Julian died of cancer at the age of 18, performing became her solace. "I threw myself into it even more. I suppose moving down here was my therapy," she says, her eyes filling up. "We're a very close family and we don't talk about it much because I'm down here. I want him to be remembered. No one should have to lose a child like that. It makes me happy that I can bring some joy to my family." On the opening night of Legally Blonde, her father walked up the pink carpet in a T-shirt adorned with Smith's face and the words, "I'm the Daddy", while her mother would bring pink, feathery coachloads down from Doncaster to watch the show every weekend. Her brother Damian, 35, lives in Gainsborough, plays in an indie-rock band called The Torn and writes songs about his little sister.

She now lives in Crouch End, North London, with her friend from school. She likes it because it's villagey and there's no Tube: "I don't think I'd be in London by choice." Nor do the bright lights of LA hold much attraction. "It's all very glamorous over there and I don't feel glamorous. It kind of terrifies me. In Britain you can have that girl-next-door thing and it's not all about beauty and having perfect teeth. I don't know that I'd go out there chasing it." She's still smarting from an American review of Legally Blonde that called her "plump and impish". "I thought, 'Oh yeah, I don't quite look like the LA girls but British audiences don't mind, do they? As long as they have a good night."

She has just completed her first movie, with Maggie Gyllenhaal. Her mother had been begging her to do a period drama but Hysteria, an 18th-century romp about the invention of the vibrator, is probably not what she had in mind. Smith plays a prostitute and has a three-minute orgasm scene. "Mum said, 'oh god, don't tell your father...'"

After Flare Path, she's keen to return to television and recently declared her ambition to be the next assistant in Doctor Who. Knowing Smith, she'll probably get her way. "I'm not one of those people who has big ideas," she says. "Then I'm not going to disappoint myself. I treat every job like it's my last. I didn't expect to come down here for three months and I'm still here. What a great 13 years."

'Flare Path', Theatre Royal Haymarket, London SW1 (0845 481 1870) to 4 June