The many faces of Sheridan Smith: The actress is taking over our TV, cinema and theatre

Sheridan Smith, "the 5ft and a fag end" powerhouse who's about to tower over us all, is recalling her first ever appearance on television. It didn't go well.

The gutsy young singer/dancer/actress, who moved to London from Lincolnshire aged 16 to star in a stage production of Bugsy Malone and never went home, was playing a chambermaid in BBC period drama Wives and Daughters. She had one line: "Bless us and save us; what's this in the bed?" But on that set on that day in 1999, she couldn't get those 10 alarmed words right. The then-18-year-old was used to performing on stage, and had the foghorn delivery to prove it. The director asked her to tone it down. Turn it down. Smith obliged. Or so she thought.

"And I still got dubbed!" she exclaims, loudly. Her voice was replaced in post-production; you can see her (if not hear her) in the offending scene on YouTube. "How embarrassing is that! One line! But I do remember the excitement of walking on to a proper film set for the first time. I always try to remember that day – so you don't take it for granted being on set. But hopefully I won't get dubbed any more."

Thirteen years on, Smith has come a long way. This month, in a staggering run of diverse projects, she's all over TV, in the dramas Mrs Biggs and The Scapegoat; on the big screen, in Hysteria and Tower Block; and on stage, in Ibsen's Hedda Gabler at London's Old Vic.

Those formative, shouty days in pantomime in Scunthorpe are back in the dusty corners of a hugely impressive and diverse CV. Smith is quieter now, more nuanced. Professionally anyway, as is clear from a CV that takes in Gavin & Stacey on the BBC, Legally Blonde and Terence Rattigan's Flare Path in the West End (for both of which she won Olivier Awards), and light-hearted middle-market telly fare such as Jonathan Creek and Benidorm. But in person she remains as gobby, funny and lively as you imagine she was back then, a performing-mad teenager from the countryside. You can take the girl out of panto in Scunthorpe…

"Nothing wrong with panto in Scunthorpe!" she exclaims. "I did that for years! Yeah," she concedes with a smile, "I was a bit hammy. But I loved it – I even loved the years playing Baby Bear, when you couldn't see my face, because I was under a big bear head! My mum still knew it was me." She shrugs. "Ah well, you learn something new on every job."

Standing in the drowsy hallway of a grand house, with her helmet of hair, mask of make-up, accent of cut-glass, foundry bell-shaped skirt and lasso of double-string pearls, Sheridan Smith looks glamorously indomitable and ready for action. It is winter 2011 and she is in Knebworth House in Hertfordshire. Smith is filming The Scapegoat, an adaptation of the 1957 Daphne du Maurier thriller that was subtitled "A novel of impersonation". Matthew Rhys plays two parts; both Johnny Spence and John Standing, the callow schoolteacher who bears an uncanny likeness to the caddish posho. Spence gets Standing drunk and swaps places with him, leaving the latter to face the music: his family's ailing business, his soppy wife, and his wife's fiery sisters – one of whom (Smith's character) is his lover.

"I hadn't read the novel," Smith says between takes, "so when I read the script. I thought, 'This is mad, this situation, these characters, the twists and turns,' but you've got to stick with it. It's one of those dramas where you have to watch it right from the beginning – you can't miss a beat cos you miss information. But it's an amazing idea – what would happen in that situation? All these mad things are happening around him, and all these crazy women, and he doesn't know what the hell is going on! Matthew is a genius the way he plays both roles so differently."

As for how Smith is playing her role, the one-time naïve Northern youngster who thought "RP" stood for "right posh" is making a good fist of her upper-class accent. "It's quite fun doing a different accent. Even when I've done the… chavvier roles, shall we say," she smiles, "I've tried to do different accents – there's Essex with Gavin & Stacey and Scouse with Benidorm. It's really good fun as an actress to have a completely different accent. It just makes you feel different straight away. So I'm all up for having a go at different accents."

Smith is sitting in the car park of a conference centre in rural Cheshire. She's wearing a mustard cardigan, stripey blouse, green skirt, brown stilettos, "and a big beehive – very Sixties! Is it comfy? Yeah, I've got used to it now. They had some fab outfits in the Sixties. I'm enjoying getting glammed up a bit. It isn't like me to be glammed up."

It is spring 2012, and Smith is in the early weeks of filming Mrs Biggs. The five-part drama tells the story of Charmian, the wife of Great Train Robber Ronnie Biggs. It's a big production, shot over 11 weeks, half of them in Australia, with Manchester doubling for Paris, Daniel Mays playing Biggs, and Smith playing Charmian as she ages from 18 through to her late-thirties.

The robbery itself takes up only seven minutes of screen time. Pretty much all the rest of the six-hour running time revolves around the traumas endured by Charmian after she falls in love with a charming, roguish carpenter-cum-petty-crook 10 years her senior: being disowned by her family, motherhood while barely out of her teens, standing by Biggs while he does jail time, discovering the truth of his involvement in the robbery, fleeing the country after he escapes from prison, fleeing another country, abortion, the death of her eldest son in a car crash, the loss of Biggs as he skips Australia for Brazil, and for another woman.

"What she went through is so fascinating," says Smith as she sits outside a busy, bustling wardrobe truck. Today she is filming scenes set in 1966, 1963 and 1957 – in that order. She's ageing backwards, and has the clouds of hairspray and expanding skirts to prove it. "I'm surprised it hasn't been done before… Charmian's an incredibly strong woman, and I'm in awe of her."

It's a huge role, and a meaningful one – Smith wanted to do justice to Charmian, who still lives in Australia and with whom she has become close during the production. It's an important part for the actress, too.

"It's a really grown-up role for me. I don't usually get to play sophisticated women – I usually do the old scrubs, chavs or slappers!" she hoots, as aware as anyone that this flagship drama couldn't be further from her breakout TV roles, knockabout comedy parts in Two Pints of Lager and a Packet of Crisps and Gavin & Stacey. In the latter she played the argy-bargy sister of James Corden's character Smithy, and for a while dated the actor.

"That's what amazed me when I got [the Biggs role]," she continues. "It's a part I so wanted." So much so that, when the call came, "I burst into tears," she admits.

"But it's hard sometimes," she reflects, "cos you're put in a box. And although I've loved all the parts I've played, it's been so nice that they've given me this opportunity. And it's scary to go out of your comfort zone, having to go to some kind of dark places cos there's a lot of serious stuff happened [in Charmian's life]. Cos it's a real-life story, you really have to try to go there [emotionally]."

Particularly harrowing is a scene set in Australia, in which Ronnie and Charmian's eldest son Nicky is killed in a car crash while Charmian is driving. "Just reading it in the audition, I was streaming – I think most people who read it were. And then at the read-through, we all went for it, it was really emotional, and Charmian was there – she was sat next to me. She must be so brave – to hear it all played back to you like that must be very surreal. I mean, to lose a child…" she begins. "I lost my brother, so I kind of think how my mum must have felt to lose her first-born."

Sheridan's brother Julian died from cancer when he was 18 and Sheridan was eight. (She has another brother, Damian, who sings in a band called the Torn.)

"You can never imagine, obviously, and I'm a 30-year-old without children. But I wanna do it justice for her, so I'm trying to get my head round that."

It's now summer 2012, and Sheridan Smith is sitting in a private members' bar in central London. She's finished filming Mrs Biggs, and is now in the thick of her next project, the new staging of Hedda Gabler. It's followed next year by another theatre project – Smith is a member of Michael Grandage's new company, and will be appearing in A Midsummer Night's Dream with David Walliams.

After spending almost two years in Legally Blonde, she's looking forward to going back to the nightly rigour of theatre. "That is the thing about a long run – the stamina," she says. "And keeping the freshness of it. Night after night, remembering that the audience is seeing the show for the first time. So you have to really dig deep. I loved doing Legally Blonde and it was a really long run. But the good thing since leaving is that I've been able to have a go at lots of different things. Hence," she smiles, somewhat embarrassed, "them all coming out in one go."

As well as Mrs Biggs and The Scapegoat, Smith appeared in the current run of Jimmy McGovern's Accused, playing an "ambiguous" nurse in the thriller alongside comedian-turned-actor John Bishop. In the film Tower Block she's a plucky woman helping Russell Tovey and other residents of a block of flats fend off a sniper (ludicrous premise, great, gung-ho performance from Smith). In the Victorian-set Hysteria, she's a patient/guinea pig of an inventor (Rupert Everett) who's created the world's first vibrator.

"I love them all," she says of her parts, "which is why I'm trying to do a mixture – if I can. I feel very grateful for having the opportunity to do so."

She's turned 31 since finishing Mrs Biggs, and is eight months into a teetotal period. Why? "I just wanted to," she says, while conceding that she remains an enthusiastic smoker. "When I was in my twenties, after I'd moved to London, I loved that whole party scene. But now with the work that's coming my ways – for instance, with Mrs Biggs: for those three months, I knew I was in every scene, and it was emotionally demanding, and it's a real-life person's story. So you don't want to let anyone down. So I just thought, I've got to knuckle down and put my work first."

But as you might expect from a working-class village girl who cut her teeth singing with her mum and dad's country and western duo, Smith is no drama-school-educated, furrowed-brown thesp: her 265,000 Twitter followers receive regular blasts of Smith's cheery enthusiasm, for her three dogs, road trips to festivals (she went to last month's V, in the company of pal Tim Minchin; Ed Sheeran and Ben Howard got the big thumbs-up), and for any musical that happens to be on telly in her north London flat.

But they also receive candid updates on her worries about her work. Recent social-media frets have included her frustration at spraining her ankle in rehearsals and whether Gabler is as "bonkers" as the actress herself. Smith is open, accessible, and not at all worried about the usual perils of celebrity. Partly, it seems, because she still isn't one. Even Corden writing about the trauma of their split in his autobiography didn't bother her.

"Me and James will always be friends. When you've loved someone you only want happiness for them." When, earlier in the summer, the actor won the Best Actor Tony Award for One Man, Two Guvnors' New York's run, "I was cheering, I was absolutely screaming. I was so proud of him. And he was doing it for the Brits – he was representing all of us over there. So, no, nothing but love there."

Did she gain any insight into the crazier side of celebrity working with Sienna Miller on Flare Path? "Oh I love that girl so much!" she shoots back. "Working with her was one of my favourite jobs. Sienna and me are friends for life. She handles everything amazingly. She's an amazing, strong woman that I completely admire."

She applauds the way Miller, and Lily Allen, stood up to paparazzi intrusion, but "I don't know about that side of things. I've never had that. Doing Flare Path, I knew that if I waited back five minutes after Sienna left there'd only be one guy left at the stage door with a camera – he just felt sorry for me! He used to take one shot. And I was like, 'Yer all right, mate, you can just go home, you don't have to make me feel better!' They're not bothered about me in that way. So I just keep my head down and do my work. But I can't imagine how horrible it must be for them. I love strong women like that – Sienna's an absolute diamond, and Lily's brilliant."

Speaking of whom… Such is the hectic nature of her work slate that it has put the brakes on the much-vaunted Bridget Jones musical. Lily Allen has been writing the songs, and Stephen Daldry was set to direct. Smith squirms when asked about the status of her involvement. But it seems that if and when her schedule and the production logistics can by synced, she'd still like to be involved.

It is, after all, a dream role for a woman with music in her blood and dance in her (albeit stumpy) legs. "I knew I wanted to be on stage somehow," she says of her childhood. "But I didn't know how. I was going to go to one of the dance colleges. But I'm so pleased I didn't, cos there's so many incredible dancers – these tall, leggy ballet dancers. That wouldn't have been me!" she cackles.

"So I am pleased that I ended up doing the acting side of things. Cos all my family are music-oriented and play instruments. So everything's just fallen into place and it's never been planned. People always say, 'What's your next five-year plan?' It's like you've got to be ambitious, you've got to think where you want to be. But I never feel like that. I've always felt that if you set yourself these high goals you can disappoint yourself. So I'm just grateful – touch wood – that I'm still plodding along and getting nice jobs. I would never want to jinx it. And I'm just really happy doing what I'm doing at the minute.

"Of course, I'd love to settle down at some point, one day," she smiles. "But I think I'd be a fool to miss out on the opportunities that I'm now being given. So everything else is on hold a bit."

So, yes, Sheridan Smith would like to apologise for her imminent ubiquity. No, she didn't plan any of it. It's pure coincidence that all these projects filmed over the past year are coming out in the same period. She doesn't mean to be in anyone's face. She just wants to keep on her own wonky but brilliant path, following a mazy route from daft comedy and light bit-parts to heavy-hitting drama, classical theatre, musical theatre, and any and all points in between. Luckily, she has the talent, energy and good humour to do the lot.

"I know it sounds clichéd but I just feel so grateful. I don't know how it's come about!" she exclaims, and it is hard not to believe her. "All I ever wanted to do was earn a living doing what I love, and then I would have taken any job that came my way. So I've been really lucky with the way it's all worked out, and how one job led to the next. There was never a plan of action. I just think, hard work. I'm a workaholic – hence, probably, why I have no personal life. I'm single with three doggies!"

'Mrs Biggs' begins on ITV on Wednesday, 'The Scapecoat' on Sunday. 'Hysteria' (15) and 'Tower Block' (15) are both released in cinemas on 21 September. 'Hedda Gabler' is playing at the Old Vic, London SE1 (0844 871 7628, oldvictheatre.com), from Wednesday to 10 November