Exactly 12 hours before I'm due to meet Sheridan Smith, the actor takes to her Twitter page. "Oh I can't stop crying. Hate myself sometimes, just can't be myself. I can only play characters. Wish I had more confidence when I'm being me," she writes. Within minutes, a stream of her some 310,000 followers are tweeting furiously.

"Omg, don't beat yourself up!" they say. "Stop stop stop that now," another instructs. One user, @Londontheatre1, has some helpful advice: Smith should draw on the strength of her characters, Ronnie Biggs's long-suffering ex-wife in the ITV drama Mrs Biggs, or Elle Woods, the ditsy sorority chick who ends up going to Harvard in the musical Legally Blonde. "Put your Charmian or Elle head on," they say.

I scroll down Smith's feed. "How embarrassing, the worst interview. I was so nervous!" her earlier post reads. Another reveals she has filmed the Jonathan Ross Show that day. "Watched it for years, never dreamt I'd be on it. I'm scared," she admits, drawing an emoticon sad face.

The next day I await Smith's arrival for lunch in a north London pub. My expectations of a bubbly pint-sized blonde – a sort of northern Barbara Windsor but with proper acting chops – are fading away. It's a busy day in the pub – two weeks before Christmas, and there is a large office party to our left having lunch – but Smith shrugs me off when I ask if we should find a quieter spot. "Oh no, no. People can never work out where they know me from," she says, extracting herself out of her fake-fur coat. "They just come up and chat with me like mates. They think they've had a pint with me somewhere."

She says this is because she has "a common face", while I suspect people fail to make a connection between Smith and her fame because she doesn't quite believe in it herself. And maybe that's because she didn't have any drama school training. None of this changes the fact that she is leaping, higher and higher, towards becoming a household name. In the past 12 months, she has worked on 11 projects (six television dramas, four films and one play).

The highlights were Mrs Biggs – six hours of gripping television in which Smith was able to showcase her impressive dramatic range – and a moment this autumn, when the 31-year-old trod the boards as the lead in Hedda Gabler, often considered the "female Hamlet". Smith, who is unusually young to be acting the part, had never come across the play, and was so overwhelmed by the gravitas of the role that she began to suffer crippling panic attacks before she even stepped out of her dressing room. Again, the reviews proved she had little to worry about. "Like many gifted actors she pulls off the apparently impossible: she makes detail dominate the stage," wrote Susannah Clapp in this paper, while the Telegraph declared: "The great thrill of the night is Sheridan Smith, revealing herself as an actress of truly tremendous talent and range."

"It started as terrible, terrible stage fright," says Smith of the anxiety that took hold, "and it still hasn't really gone away. But what did I expect? It was a part that required me to go on and shoot myself eight times a week, to have a full-on breakdown each day."

Since finishing the eight-week run, she has barely taken a break. The morning after her last show, she went straight on to filming Panto!, the ITV comedy drama written by comedian John Bishop which aired three days ago. "It was just the tonic I needed – something a bit fun and silly," she says. Wasn't she tempted to have a holiday? "People did say that, but I just thought: 'Where am I going to go? Who am I going to go with?' I live on my own, with my dogs. And it was so much fun anyway."

After Panto! she hit The Powder Room, a London-based film about a group of female night-club toilet attendants that will be released next year. Again, there was no time to recuperate. "I remember them scrubbing off the fake tan I had to wear for Panto! and the next day I was on a movie set at 6am." She smiles when I look aghast. "It is so surreal," she says. "It is so… schizophrenic. You sort of forget who you are, in a way."

She admits to becoming something of a "nervous wreck" as we approach the year's end, which half explains the Jonathan Ross situation. "I just feel like I really ballsed it up, you know?" she says, making a "Have I?" face. At the time of writing, I genuinely don't know – the show was aired after my deadline – but she says: "Oh well, I've gotten over it now anyway. But I was a bit baffled by being asked to go on it in the first place. I didn't want to do it because I'm shy, insecure, and I feel like you have to have funny stories and anecdotes on those shows."

I point out that the very fact that Ross invited her on alongside Paralympians Ellie Simmonds and Jonnie Peacock, two of the greatest achievers of the past 12 months, suggested she deserved her place. Her ever- rising star, and the fact that she is so widely loved, is why we've put her on our final cover of the year, I say. "I know. But it's odd. It's like, the more successful… the bigger the parts I get… it's like I'm going to get found out. Because I've always felt I'm blagging it."

As sMITH is THE daughter of a country and western duo, there was always the possibility that she herself would want to perform from a young age. At the age of six she began to join her parents, known as the Daltons, on stage. "They grafted really hard," she remembers, "all over Lincolnshire. It was seven nights a week and that was the only job they did." By the time she was 11, her school drama teacher had advised that they take her for an audition at the National Youth Music Theatre and Smith ended up performing with them during her summer holidays, flying to New York first for a show called Pendragon, before taking parts in Bugsy Malone and Stephen Sondheim's Into The Woods in London.

At 16 she decided to quit school and moved into a house-share in the city with five other girls, who included S Club 7's Hannah Spearritt. "We were so young!" she remembers now. "We had no money, we couldn't get into a pub, and we couldn't cook, so we lived on jam sandwiches."

It is a credit to her ability that she has found herself in a "normal job" only once over the past 15 years – in a burger van off the motorway, which she "loved, because we served loads of bikers and I could say: 'Do you want sauce with that, love?'" She hams up her weathered Lancashire accent, and you sense she was playing dress-up even then, always learning on the job, in a sense. "Because that's what I did. I wasn't trained."

In the formative years of Smith's television career, on sitcoms such as The Royle Family (in which she played Ralf Little's girlfriend) and Two Pints of Lager and Packet of Crisps (in which she starred for eight years) she didn't feel like an imposter at all: "I was just plodding along like a jobbing actress, and it felt comfortable." Now it bothers her that she didn't go to drama school. "My agent and I made a conscious decision that this year I would try and break into drama, but I honestly didn't know if I could do it," she says. "Of course I've surprised myself, but it feels strange. If I was to describe it, I feel like a little duck gliding along, with my legs going faster than my brain."

She believes it was Flare Path, Trevor Nunn's 2011 theatre production, for which she won her second Olivier award (the first was for Legally Blonde), that lead to offers of more serious roles. She had mainly played "slappers and chavs in comedies, and although it was great, you can get typecast," she says. The play, which was supposed to be a vehicle for Sienna Miller's glittering stage career, saw Smith steal the show, winning rave reviews, and one night, a visit from a certain Dustin Hoffman to her dressing room.

"He came to tell me how he thought I acted from the heart," Smith says. Some time later he invited her to appear in Quartet, his directorial debut about a group of retired opera singers (played by Dame Maggie Smith, Pauline Collins, Michael Gambon and Billy Connolly) that will be released on New Year's Day. It's a gentle tickler of a film that will appeal to the crowd that turned out for The Exotic Marigold Hotel, but after Smith's stellar year her part seems small, not making the best of her talent at all.

In 2013 she will tackle Shakespeare, starring as Titania in Michael Grandage's A Midsummer Night's Dream opposite David Walliams, who cast her in the recent BBC adaptation of his children's book Mr Stink. "I'm a sucker for punishment, aren't I?" she says of the six-month run which will start in the summer, "but I'm just so grateful that people believe in me. It's not like I'm choosing these things, even saying I can do them. People are just being lovely and saying: 'I bet you could challenge yourself to this.'"

She is, she admits, at risk of never stopping, with work for the first half of next year also lined up (a film and a television project that she can't talk about now). It would, she realises, be scarier to stop than to just keep going, although at some point she'll need to find a work-life balance because she would like to get married and have kids. At the moment her personal life is "nonexistent". She isn't dating but "I wish I had someone," she sighs. She is coy when I ask her about being photographed stepping out with Amy Winehouse's ex Reg Traviss earlier in the year, dismissing it as: "Nothing. Nothing at all. We're just mates. But I'm enjoying kissing a few frogs, let's say."

She has been in a "couple" of serious relationships and dated James Corden on and off for two years but: "A lot of things are about timing, aren't they? I was a lot more wild back then, too. I wasn't someone who was in a frame of mind of settling down when I was with James." She didn't attend his recent wedding, but they are still friends. "I think when you've loved someone that much it doesn't ever go away. It wasn't right, but I knew he would be a fantastic husband and dad, and I want the best for him always."

She still describes playing Corden's screen sister Rudi in Gavin & Stacey as one of her most fun roles to date, and I tell her my favourite scene of the whole series has to be the rap they perform together in the car park, complete with the intricate handshake. She laughs. "I think we had been practising that on holiday! We had been in Mauritius and we were by the pool doing it every day."

Her sense of comedy probably comes from her dad. At a recent event where he was Smith's guest, he told the press that no one would ever be good enough to walk his daughter down the aisle, so that's why he was walking her up the red carpet. She describes the Smith clan – which also includes her mum and older brother, who is in a band – as "tight-knit", citing the death of her brother Julian to cancer when Smith was eight and he was 18 as the reason for being closer than most. Later she will tell me how she has started to "use" her grief in her crying scenes, something she feels "terrible" about. "But also happy," she adds, "because he is living on, somehow, in my performances." This, she feels, is better than trying not to confront his death at all, which she did for many years: "And it was odd, because I started to feel like I was denying him."

We go on to talk about a grief scene in Mrs Biggs, and it strikes me that out of all the projects Smith threw her heart and soul into this year, this is the one of which she is proudest. "It was such a special job, the best role I might ever have, and so I worked really hard on it. I gave up drinking," she adds just as the party next to us start to clink their glasses. "I just thought that if I'm in every scene of something I won't drink – I can be a bit of a party girl – and you know, I've stuck with it."

She goes on to reveal that this now means she hasn't consumed a drop of alcohol all year. And it is only later, when I'm transcribing our interview, that I remember: when Smith arrived at the pub, she sat down and spent some time really deliberating whether she should treat herself to a glass of red wine (she didn't). That tells me that, as the remaining wintry nights of 2012 draw in, maybe this sensitive, self-effacing soul is beginning to take stock of all she has achieved this year and finally start celebrating.

Quartet is released in cinemas on 1 January