With a rare ability to swivel between comedy and drama, Jane Horrocks is too good an actor to have been typecast. But most of her iconic roles have been coloured with the joys, pains and daftness of youth: her break-out role as Nicola, the petulant teenager in Life is Sweet; Bubble, the vacuous, saucer-eyed PA in Absolutely Fabulous; even Prunella Scales's grown-up, yet bossed-around daughter in the Tesco ads. Then there is Little Voice, the singing prodigy she played on stage and screen that won her Olivier, Bafta and Golden Globe nominations.

When we meet at the Young Vic, the painted face, floral blouse and blonde bob are still girlish, doll-like even, and her voice - still boisterously Lancastrian after two decades in London - retains that familiar squeaky register. But, during rehearsals of a new production of Bertolt Brecht's The Good Soul of Szechuan, Horrocks clearly feels she should be acting her age (she's 44).

She and her partner, the television writer Nick Vivian, have two children, 9 and 10, and she insists, reasonably, that “you have to be a bit more grown-up and mature when you've got kids”. Questions are played with a straight bat. When I remind her of such former statements as “I don't think you should act when you get old” she grimaces. “I don't know why I said it - I just wanted to be perverse.” So she's carrying on? “Absolutely. When you get older different doors open, more intricate parts.”

So, bar the odd voiceover, the Tesco ads are over, and it's probably goodbye to Bubble, too. “The older I got, the more difficult I found it to play her,” she admits. “It just didn't feel right because I had joined the grown-up world.”

She certainly displays a fogeyish attitude towards technology: “I don't have a computer; I don't know how to turn one on. I don't do e-mails.” It infuriates her friends, not to mention Vivian, whose IT skills she relies on in moments of need. “I annoy him to death when I have to do research,” she admits.

Taking the dual lead role in one of Brecht's great moral parables is another potent way of proving her membership of the grown-up club. Horrocks plays Shen Te, a compassionate prostitute who is tested by three gods who want to discover whether there is any good left in humanity. Shen duly helps the poor and needy, but soon becomes exploited by her less scrupulous peers. She is forced to pose as a hard-hearted male cousin, Shui Ta, to protect her interests. For Horrocks, the play is about “whether you can be good, and remain good, in a corrupt world - and you do come out of it thinking that you can't”. What she calls “women's stuff, and the emotional baggage that comes with that”, she says, “gets in the way of getting on in life. I don't think the world has changed at all.”

She approached the production with a refreshing absence of dramatic baggage. She is not political, she says, and when I ask whether she rates Brecht, as many do, as the most important figure in 20th-century theatre, she shrugs: “I don't know enough about his work to comment.” Nor does she express a particular interest in the last high-profile production of the play at the National Theatre, in 1989, starring Fiona Shaw. “No, I haven't seen that one so I had absolutely no knowledge of it.”

She thinks that her director, Richard Jones, wanted her “because I've done comedic things, even though I don't think I'm very funny”. She chuckles. She took on the role after finishing a praised run in Alan Ayckbourn's quasi-farcical Absurd Person Singular, “but I don't think there will be any laughs in this. Well, there don't appear to be any so far, but you never know until you put it in front of an audience”.

Where a facility for comedy comes in handy is in rounding the edges of Brecht's often hectoring role plays: “My character has quite a lot of proclaiming to do, and to make that non-preachy is quite hard. I think the director was trying to soften all the things that put people off Brecht.”

Central to that process is the talent for mimicry that has served Horrocks throughout her career. Bubble was inspired by her five-year-old niece, and Little Voice by her dressing-room re-creations of standards by such grand dames as Shirley Bassey, Ethel Merman and Peggy Lee.

By that time she was in a year-long relationship with the late musician Ian Dury, which she describes as “not boring!”

“I remember him coming to see Little Voice on stage and saying that my singing was a bit dodgy. I said, ‘Oh right, thanks a lot!'” Most disagreed, and she is proud to consider it as her defining role: “It encapsulates everything I can do.”

She has based the character of Shen Te, she says, on a close friend. Is she Chinese? “No, she's from Nottingham,” Horrocks laughs. “But I really felt like I totally inhabited her.” The rehearsals have reminded her of the improvisations she used to do with Mike Leigh, “because Richard was just sat in a corner, listening to it. I really missed those improvisations we did for Life is Sweet. You would just be given a situation and then you'd improvise with the other person.”

I wonder, slightly crassly, whether it was those sessions that produced the film's infamous scene - in which Horrocks's character and her lover coat each other with chocolate spread. There is a cold stare and what can only be described as a matronly thinning of the lips. “I think we'll leave that one.”

It's at moments like this that you are reminded that Horrocks's background does not fit comfortably into the template of the gushy, soul-baring actor. Born Barbara Jane Horrocks in Rawtenstall, Lancashire, she came from a Methodist background in which her grandparents were “very big in the church”. There was performance, although it was mainly restricted to singing in her local congregation.

But she won a place at RADA, from where she skipped straight to the RSC. Her RADA contemporaries included Ralph Fiennes and Imogen Stubbs: “I still see Imogen every now and then, whereas I don't see Ralph at all, apart from on television and film.” She giggles. “No, not on television. He doesn't do television, does he?”

No such qualms for Horrocks, whose recent small-screen appearances include The Amazing Mrs Pritchard, in which she played an ordinary woman who is elected prime minister; and Hunting Venus, which was directed by her friend Martin Clunes (her successor in the Tesco ads) and written by Vivian, her other half. “A load of mates were in it, so it was really jolly.” She also hopes to appear in another project by Vivian, “which I'm not going to talk about”.

Theatre, however, is what she's enjoying most at the moment. It hasn't always been that way - after an eight-year break to have children, her appearance in Steven Poliakoff's Sweet Panic in 2003 led her to announce that she didn't want to do any more theatre. She grimaces again: “I don't know why I said that.” Sure enough, she missed it, and has resolved to avoid long gaps in future - there was only a week between finishing the Ayckbourn play and starting on The Good Soul of Szechuan. “I don't think it's a good idea to leave the stage for too long because you get frightened,” she explains.

But the Young Vic's coffers are far from bottomless, so she is happy to use those chirpy tones in voiceover work to pay the bills - and not always particularly grown-up stuff. “It gives me a licence do things like Good Soul,” she says with a wonky smile, “knowing that I can earn a bit of money doing Fifi and the Flowertots.”

The Good Soul of Szechuan previews at the Young Vic, London SE1 (020-7922 2922) from Thursday, and opens May 14

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