As might be expected from a Mike Leigh film, Blethyn's role is by no means a glamorous one. She plays Cynthia, a lonely and rather sad single mother with a dull factory job, whose life is turned upside down when her first child, adopted at birth, comes to seek her out. Nor are the circumstances of our meeting glamorous. Blethyn is preparing for next month's opening night of Sam Mendes's revival of the Alan Bennett farce, Habeas Corpus, so we schedule the interview for a rehearsal room in Waterloo.
Blethyn, a tiny figure in a long raincoat slung over casual rehearsal clothes and no make-up, is waiting for me in the street with the news that the venue is now locked for the night. As we wander through the streets of Waterloo looking for somewhere to chat, Blethyn entertains me with stories of how she and Marianne Jean-Baptiste, who plays her daughter in Secrets and Lies, reacted to the hullabaloo of their first Cannes. She continues as we settle in the foyer of the National Theatre, where she established her career in the Seventies.
"It was absolutely thrilling. Marianne and I were trembling in the limo on the way to the gala screening. Then it stopped at that long flight of stairs with the red carpet and as we got out we looked at each other and went, 'Wow, this is great!' As we were going up the stairs, there were banks of photographers either side and TV cameras and everybody was shouting nice things. It was just dreamlike, really."
Leigh's working methods - in which the shape of the piece is governed by the way its characters develop both individually and then interactively as relationships progress - were not new to Blethyn. She worked with him 16 years ago on his television play Grown-Ups and was not fazed by the intensity of the experience. Even so, working on Secrets and Lies was not without its surprises.
"Marianne and I didn't know each other before this, so my character, Cynthia, had no idea what Hortense was going to be like. I'd only spoken to Marianne on the phone, didn't even know she was black. The first time I met her was when Cynthia met Hortense outside the tube station."
The fact that her daughter's skin colour comes as a shock to Cynthia is only half-explained in the film, with her conception remaining as one of the character's "secrets". When pressed on the point, Blethyn fidgets nervously, as though being asked to betray the confidence of a friend.
"It's rather delicate. Cynthia chooses not to explain it and I just don't think that I should."
Blethyn is best known to the general public for her television work; Christopher Morahan's The Bullion Boys, for example, and the controversial serial The Buddha of Suburbia, and for her role as cricket widow Miriam in the sitcom Outside Edge, which also starred Timothy Spall, who plays her younger brother in Secrets and Lies.
It has not been a career entirely defined by mumsy TV roles, however. There has been plenty of substantial theatre work, ranging from Nora in A Doll's House, through the Restoration comedy canon with such roles as Mrs Sullen in The Beaux' Stratagem at the National, Mrs Cheveley in An Ideal Husband, a touch of 20th-century glamour as Billie Dawn in Born Yesterday and an award-winning performance in Steaming, in which she didn't wear very much at all.
Blethyn came comparatively late to theatre. Born and raised in the Kent seaside resort of Ramsgate, the daughter of a mechanical engineer, she left at 17 to work in London as a secretary. As a practical young woman, the theatre had never seemed to her a sensible option as a career. But after working with amateur groups in her early twenties, she decided, in her own words, "to have a go", auditioned for the Guildford School of Acting and got in. Her first professional engagement was a season with London's touring "theatre in a tent" company, The Bubble, followed by the Belgrade, Coventry and small parts at the National Theatre, where her life changed in two significant ways. She met Michael Mayhew, the designer of the theatre's posters and programmes, who became her long- term partner (whom she describes rather more colloquially as "my boyfriend"). Then she got her big chance when Kate Nelligan left the cast of Tales from the Vienna Woods. Blethyn stepped into the role.
The forthcoming Habeas Corpus at the Donmar Warehouse - the first production of the play for 20 years - is in some ways a contemporary equivalent of the Restoration comedies of lust that Blethyn has enjoyed so much in the past. Blethyn and Jim Broadbent play Muriel and Arthur Wicksteed, whose marriage "has lost its romance". The moral of the story? "He whose lust lasts, lasts longest."
"It's rather like one of those seaside postcards," says Blethyn. "It's a terribly funny play and the wit is very much Alan Bennett. He has his own language."
After Habeas Corpus, which could follow the Donmar's successful production of Company into the West End, Blethyn has no firm plans, though she will probably be involved in promoting Secrets and Lies for its opening in America. She has just completed a Michael Frayn-scripted film, Remember Me, for Outside Edge director, Nick Hurran, and she and Jim Broadbent will soon be seen in a series of television monologues under the title First Signs of Madness. But even though the signs are that Blethyn is going to be hot property for a while, this practical woman of Kent is not letting it go to her head.
"Touch wood, I haven't been out of work. I work hard, turn up on time and know the lines, but there's a lot of people could do that and I'm lucky that it's me."
n Brenda Blethyn stars in 'Secrets and Lies', on general release from Friday, and 'Habeas Corpus', playing at the Donmar Warehouse, London WC2 from 30 May to 27 July. Booking: 0171-369 1732