It's been emotional: Lesley Manville on banned sex scenes and how Mike Leigh turned her into a regretful alcoholic

You hate to typecast actors, but sometimes they don't mind doing it themselves, by way of self-mockery. On the website of an agency specialising in voice talent, I came across a sound file of Lesley Manville introducing herself in jokily crestfallen tones: "I'm very sociable... Last week I went out almost every night – mainly to the garden." There couldn't be a more concise way to tell the world that you're a working British actress, down to earth, GSOH.

"40-50s, Confident and Approachable RP," reads her tag on the site. Confident and approachable – yes, in person, Manville is certainly that, and the vowels fairly approximate to Radio 4-style Received Pronunciation. But the formula hardly covers the Lesley Manville you've seen on screen or stage. Accent-wise, she's done everything from the brittlest British cut glass to tremulous estate cockney, via uptown Manhattan. As for her characters, some are neither remotely confident nor approachable; her latest creation, in Mike Leigh's Another Year, is an out-and-out car crash of a woman.

A Leigh regular since the late 1970s, Manville has over recent years been centre stage in several of the emotionally decisive scenes in the director's films. In Topsy-Turvy (1999), about Gilbert and Sullivan, she had a memorably surreal and haunted monologue as Mrs Gilbert. In All Or Nothing (2002), as a woman electrically crackling with sorrow and resentment, she had an austere final-act scene with Timothy Spall, guaranteed to leave viewers as ashen-faced and trembling as her character. Now the 54-year-old has her most-developed Leigh role to date. In Another Year, she plays Mary, a middle-aged woman with a chronically mismanaged emotional life. It's a hugely powerful, sometimes crisply funny performance; many critics felt she was robbed of the Best Actress award in Cannes this year.

Manville is finding it hard to avoid the spotlight. Having started 2010 at the Old Vic in Six Degrees of Separation, she'll end it on the Oscars trail for Another Year: Sony is distributing the film in the US, and will be touting it in this winter's awards campaigns. Manville is preparing for a strenuous stint on the American PR circuit, with some trepidation. "I keep saying, 'What is an Oscar campaign, what's going to happen?' I don't know what I'm going to have to do – be there, I suppose, be nice, be talkative," she says, sounding rather like a head teacher anxious before her first open evening. She's excited by the possibilities that the film's exposure could open up – in a very pragmatic way. "I could go and work with some of the interesting people in America. I could go and make some money – because, Jesus Christ, you can't earn any money here any more. I work for the BBC now and I'm getting... I haven't worked it out, but it feels like almost a third of my fee from 15 years ago."

There's just enough controlled agitation in Manville's generally relaxed, voluble manner to see where she might source the tension in some of her characters – although she's light years from the angry-sparrow agitation of Penny in All Or Nothing, or the stretched-wire bonhomie of Another Year's needy Mary. But these characters shouldn't be reduced to capsule formulae. Products of the famously exacting ' Mike Leigh method, they emerge from painstaking elaborations of personality and biography.

"A lot of people think you make it as a cold decision – that you just decide you're going to play this lonely alcoholic who's never had children and has lots of regrets. What Mary is on screen is very naturally and organically what she became because we created [her back story as a] child, the young girl who never knew her father, who was a bit of an outsider at school, couldn't keep friends, and then would have a series of disastrous relationships and marriages. When you get to the point where you start improvising them in their 'now' existence, all of that earns its keep."

Manville was first recruited by Mike Leigh in the late 1970s, for a project at the RSC. She then worked with him on a radio play that wasn't broadcast by the BBC for 13 years – because of her sex scene with Phil Davis, deemed too realistic for radio – then in his 1980 TV film Grown-Ups. "When you're working with Mike, it demands every bit of you. You're not just employed to learn the lines and turn up, having got the character together on your own at home. He is employing you to work with him, and Mary is as much a creation of mine as his."

Among all her roles for the director, there is one which is probably the most contentious in Leigh's catalogue. "I know what you're going to say," Manville says, laughing. It's Laetitia Boothe-Braine, the ghastly embodiment of urban gentrification in 1988's High Hopes – all contempt and crisp vowels ("Chop chop!"). Laetitia works fine as a broad Dickensian grotesque – but she's surely not plausible in terms of any known reality, even of the Tory 1980s? Manville begs to differ. "When David Bamber [who played Rupert Boothe-Braine] and I were researching those characters, we spent a lot of time with some seriously posh people – not just upper middle-class, but seriously posh – and I came away feeling that we could have gone so much further," she says, stifling a laugh. "Honestly, the amount of time I spent in those selective elitist corners of Harrods, where women were like Laetitia with knobs on..."

In fact, Manville has a formidable line in conservative women – last year, she was a deeply insecure Margaret Thatcher in Channel 4's drama-documentary series The Queen. "I was very pleased with my Margaret, even if I say so myself – I particularly got the walk and the handbag. I just loved getting the looks right, and the ego, even when she's against the Queen – the status that she was pulling with the Queen!

"Lots of people have said to me, 'Oh, can't imagine you playing Thatcher.' Well, that's like bait for me. But I think, people don't say, 'We need someone working-class, let's get Lesley', or 'We need somebody posh, let's get Lesley.'" Her casting, she says, goes "anywhere, any distance, really".

Manville herself comes from a working-class background in Brighton and Hove, where her father was mostly a cab driver and generally, she says, "a very charismatic jack of all trades". Because he gambled, there would be sudden but temporary influxes of wealth: "There were times when he had money that gave us this kind of other existence." Her parents "were both very, very glamorous, and my mother loved clothes. On Sundays they would go to hotels and meet up with their friends, and it would be gin and tonics. There's a bit of footage we've got at home, on this old 8mm movie, of them dancing in this club – it's all a bit dark and moody, and cigarettes."

Manville's first love wasn't acting but singing – she was twice champion soprano of Sussex. Everyone assumed she was destined for opera, but she discovered drama when she went to the Italia Conti theatre school and suddenly found herself improvising. "I remember that moment, I suddenly thought, 'Oh this is good, I can do this, I like this.'" Arlene Phillips, who taught there, invited her to join a dance troupe she was forming. It was Hot Gossip, and Manville shyly declined: "I just thought, 'I can't be on telly in stockings and suspender belts – my dad would just be so embarrassed.'"

Professional work came quickly. She got her first job in a West End musical directed by John Schlesinger; presented a children's TV programme; did four episodes of Coronation Street, playing a character she described at the time as "a gormless twit". And she had a long, secure run on what was then Emmerdale Farm. "The truth is, I didn't have an idea about myself, what I wanted to do, what kind of actress I wanted to be. I went to Leeds Grand and played Cinderella, so it was 'Lesley (Rosemary in Emmerdale Farm) Manville' – it was brilliant. I was earning so much money and I was 19; it set me up for life. Artistically I didn't have a thought in my head. And then I met Mike Leigh. And then I started working at the Royal Court."

It was in theatre that Manville met Gary Oldman, the father of her son Alfie, now 21 and recently graduated in film studies. Manville and Oldman had been together for some years when the director Alan Clarke cast them both independently in a BBC film that remains one of the hardest-hitting pieces of British 1980s drama – The Firm, about white-collar football violence. Oldman played Bex, captain of a London football gang; Manville, his wife. By the time the shoot started, not only were the two actors married, but Manville was four months' pregnant. They decided not to tell Clarke. "There were a lot of scenes that were rough and tumble and we didn't want him to worry about that. In fact there was a major scene that was cut, which I've seen only once. Bex comes back from a football match pumped up, and he starts throwing her around, throws her on the floor, hitting her, rapes her – and then she starts laughing and really gets into it. It's a frightening scene, because you realise that this violence is a mutual sexual thing that they had going on – and of course, the BBC just wouldn't have it."

The couple's marriage was short-lived; Manville was later married to the actor Joe Dixon, but is now single. For the most part, she raised Alfie herself; she says it's what helped her clock off at the end of a day working on Leigh roles. It also, conceivably, gave her that edge playing mothers, which have come in every shade – including a Madrid matron (in the Old Vic's Almodovar adaptation All About My Mother), plus a consistent flow of variously concerned or feckless English mums. She recently shot Womb, a bizarre Hungarian-directed film about love and cloning, as mother to Doctor Who's Matt Smith. In fact, she's had a run of massively cultish Brit boys as her progeny – others being James Corden in All Or Nothing and Colin Morgan, the BBC Merlin, in All About My Mother. "And you know what?" she says. "All those boys have done rather well since they worked with me as their mother. They owe me big time. They're all wealthier than me – they can have a whip round."

'Another Year' (12A) opens on Friday