Ditching her Gavin and Stacey tracksuit for the girlie glamour of the West End, Sheridan Smith is starring in Legally Blonde the Musical. It's a fresh chapter in her life that has brought not just critical acclaim, but also romance. Here, for the first time, she talks about the new man in her life

'I was so worried that no one would take me seriously after years of being typecast as a tomboy,' says Sheridan

Ten minutes into my interview with actress Sheridan Smith, and I’m still struggling to reconcile this soignée, stylish young woman, hailed as the West End’s classiest new star, with her trashy, tracksuit-bottomed television alter ego.

‘I know, I know just what you mean!’ she chirrups delightedly in her high Northern voice. ‘I usually play slappers, and if someone told me that the chavvy bird off Two Pints of Lager and a Packet of Crisps had been cast as the all-singing, all-dancing lead in Legally Blonde, I’d assume she’d be rubbish, too.’

But here’s the thing: Sheridan, 28, isn’t rubbish, she’s utterly sublime – glamorous and sassy, with a powerful pair of lungs, a fabulous figure and impeccable comic timing. Janet from Two Pints – aka ‘Our Ant’ney’s’ girlfriend Emma from The Royle Family, and Rudi, the gobby sister of Smithy in Gavin and Stacey – gives a flawless performance as high-maintenance Californian rich-girl Elle. Sheridan so completely inhabits the role created by Reese Witherspoon in the 2001 film on which the stage musical is based that I didn’t think of Witherspoon once; or maybe just the once, to marvel at how I wasn’t thinking of her.

Her transformation from superficial airhead to legal eagle, via 18 songs, 19 costume changes and two magnificent beach-blonde wigs, has won Sheridan a standing ovation at the Savoy Theatre every night since the curtain went up in January.

‘I prefer theatre to television – you get to feel the love,’ she says, without a hint of sarcasm. ‘I spend up to an hour every night after the show signing autographs and posing for photographs and feeling so touched that people are enjoying the show. The actor David Tennant wrote to say how much he liked it, and choreographers Arlene Phillips and Jason Gardiner [from Dancing on Ice] both came backstage and were so lovely to me that I had to pinch myself.’

Today, dressed in a pretty Alice Temperley polka-dot strapless dress over thick tights and biker boots, Sheridan has a rare freshness about her. When we meet, she gives me a hug and plants a hearty kiss on my actual cheek, rather than the industry-standard air kiss. She also touches my arm repeatedly with can’t-help-herself enthusiasm and exudes a diva-next-door warmth that makes her seem younger than her years.

From left: with her ex-boyfriend and G&S co-star James Corden; her new beau, actor Ross McCall

‘I’m a very tactile, open person, and when I moved to London on my own as a teenager, I would get into tricky situations because guys would think I was coming on to them, rather than just being friendly,’ she says.

‘As a result, I developed quite a tough, tomboyish exterior, which was reflected in the roles I got. Appearing in Legally Blonde has helped me find my inner girl, although at the beginning the director was constantly telling me off for sitting like a boy, with my legs apart, while wearing a cocktail dress and heels! I’ve even let pink start creeping into my wardrobe – I used to wear nothing but jeans and a T-shirt but now I’ve got a shocking pink bag from Topshop and a gorgeous pair of pink and lilac suede Nicholas Kirkwood shoes.’

For the uninitiated, the Legally Blonde film is the sine qua non of chick-flicks; a candy-pink soufflé of girl power and to-die-for accessories. Bubbly sorority girl Elle, who is majoring in fashion merchandising, is abruptly dumped by the love of her life when he wins a place at Harvard Law School, citing a need for a more serious girlfriend. So she resolves to follow him, confounds expectations by breezing the requisite exams, takes on the snobbish might of the Ivy League establishment, complete with It-girl tiny dog, and wins hearts and minds along the way.

‘I’ve heard people questioning whether a dizzy blonde with a chihuahua in a designer handbag is really a good role model,’ says Sheridan. ‘If there’s a message, it’s that a lot of shallow people will judge you on how you look and speak, but you should remain true to yourself regardless. When it comes down to it, it’s two and a half hours of entertainment, not Chekhov or Shakespeare. The audiences want feel-good escapism from the weather and the economy, and that’s what we’re giving them.’

Sheridan was born in Epworth, in Lincolnshire, to parents Colin and Marilyn, a country and western duo who performed as the Daltons. It was watching and occasionally appearing on stage with them that sparked her passion for performing, and she successfully auditioned for the National Youth Music Theatre. She was cast as Tallulah in Bugsy Malone, which had a run in London when she was 16. Instead of going back home, she was signed up by an agent and moved into a flat with her fellow cast member Hannah Spearritt, who went on to be in S Club 7.

‘I’m really grateful to my parents for having the confidence in me to let me go,’ she says, in retrospective awe. ‘I was terrified I might have to slink back to the village with my tail between my legs, and treated every job as though it were my last – I still do – but fortunately I got work and things seemed to slot into place.’

A part in Stephen Sondheim’s Into the Woods brought Sheridan critical acclaim; in 1999, when she was 17, a prescient broadsheet article about the newest crop of talent to watch photographed her alongside a puppyish Jamie Oliver. Parts in The Royle Family and BBC3’s Grownups followed. Then came ITV comedy Benidorm and a role in BBC1’s Jonathan Creek. Sheridan was also cast in Gavin and Stacey, and embarked on a high-profile on-off relationship with James Corden, who played her brother, Smithy, and co-wrote the show.

Although early in 2009 Corden was reportedly talking of marriage, the couple went their separate ways towards the end of last year, but still remain friends. Reading between the lines, Sheridan’s single-minded dedication to her craft may well have been a factor in the split, as she admits that she was filming constantly in the run-up to Legally Blonde rehearsals, and that once they began, she could think of nothing else.

‘James is very proud of me and it’s entirely mutual,’ she says with real affection. ‘He’s an amazing person and I had a great time being with him. It was sad that things didn’t work out, but I wouldn’t ever want to lose him from my life and I hope he’s happy.’

‘I regularly bring home each of the five chihuahas who appear in the show and sleep in bed with them, to bond’

I ask Sheridan if she fancies Duncan James, the TV presenter – of ex boy band Blue – who plays Warner, the handsome but spineless boyfriend Elle pursues then rejects in a glorious, punch-the-air moment of self-determination. ‘Of course I do, who wouldn’t?’ she responds. ‘I get to kiss him every night on stage, which is very nice, although he winds me up by slipping the tongue, and it’s a real struggle not to giggle.’

But any flirtation with her leading man can be deemed harmless, because there’s another reason for Sheridan’s joie de vivre. She’s dating, or as she euphemistically puts it, while blushing furiously, ‘hanging out a lot with’ Scottish actor Ross McCall, whom she’s known for years. Based in Los Angeles, where he appears in the US television series Crash, McCall, 34, who was once engaged to Jennifer Love Hewitt, is currently in the UK and staying at Sheridan’s North London flat.

‘Ross and I first met ten years ago in London, but lost touch. Then he looked me up on Facebook a couple of months ago and we got together and, yes, I know I have a silly smile on my face, but I can’t help it,’ she says. ‘He’s great; handsome and charming and kind and he makes me laugh. He also gets the fact that I’m totally focused on my work, because he’s the same way.’ Later, McCall turns up to watch Sheridan’s YOU photo shoot, and the pair are as unguardedly smitten as teenagers. ‘And one of the best things is that Ross has a dog, so he isn’t at all fazed by the fact I have three – or that I regularly bring home each of the five chihuahas who appear in the show and sleep in bed with them, to bond.’

Sheridan says this as though it were perfectly normal. Then she explains she owns a shar pei christened Enid, a Japanese spitz called Trish and a pug who goes by the name of Barry Manilow, by which time a rather sweet streak of eccentricity becomes apparent. ‘I’m a bit of a slack mum – I’m hopeless at discipline, so they just run riot,’ she sighs, with exasperated indulgence. ‘Barry Manilow’s the worst; he tears up the upholstery, and since Ross came along he’s been cocking his leg to mark his territory: Barry, that is, not Ross – he’s completely house-trained.’

Sheridan as Elle in Legally Blonde the Musical, opposite Duncan James, formerly of boy band Blue

Although it may be tempting to infer that Sheridan has led a charmed life, her success has been tinged with tragedy. When she was eight years old, she watched her eldest brother, Julian, grow frail and die of cancer, aged 18. ‘People think that little kids don’t notice what’s going on around them, but I remember the whole sequence of events vividly, like it happened yesterday.’ It’s a subject she has only recently talked about in public, and her eyes pool with tears as she speaks.

‘Julian would spend more and more time just lying on the couch and I would do little shows for him. When I was being an annoying little sister, he would still find the strength to hold me at arm’s length, laughing as I flailed around punching the air, trying to get at him. I wish he was still here to see me. Maybe he is. The upshot is that it’s made me want to achieve all I can and make the most of the opportunities I’ve got, because I’ve seen how precious and fragile life is.’

Sheridan is close to her other brother, Damian, 34, who plays in indie rock band the Torn. When she goes to his gigs, he plays a song he wrote for her, called ‘Wild Girl’, which always makes her cry. It’s evident that the whole family are supportive of one another. ‘On the gala night of Legally Blonde, my dad walked down the pink carpet wearing a T-shirt with my picture on it and the words “I’m the Daddy”. My mum is bringing two coachloads of aunties, uncles, cousins and friends down to see the show soon; they call themselves “SAS”, as in the Sheridan Appreciation Society.’

It’s a fan club with a rapidly growing membership; reviewers have variously described the show as ‘ridiculously enjoyable’, ‘perfection’ and ‘hysterically funny’. Sheridan herself wins hyperbolic praise. Meanwhile, she insists she’s looking no further than the end of her contract in October, although bookings are being taken until February and it beggars belief that anyone else could play the role Sheridan has made her own.

‘It’s hard work. At the first rehearsal we were all made to skip for an hour to get our stamina up. I lasted about a minute because I was the sort of girl who got a stitch walking to the car. Now I’m so fit I’ve gone down from a size 12 to an eight and can eat burgers every day because I’m burning it all off,’ she says, patting what passes for a tummy.

‘I love being part of the show; I was so worried that no one would be able to take me seriously after years of being typecast, and that I would be shot down in flames, but I was wrong. Not having gone to drama school, I always feel like a bit of a fraud, but so far it looks as though I’ve not been found out.’ Quite the reverse: Sheridan Smith has been discovered. The chavvy bird has turned into a beautiful swan and is gracefully stretching her wings while we all look on in wonder.