I meet Allison Janney at the Beverly Hills Hotel where, the previous day, the Clintons were spotted dining in the Polo Lounge. This feels like a diary mix-up: as CJ Cregg, Janney served for two terms as White House press secretary and chief of staff and should surely be on a campaign bus with them somewhere. She smiles and rolls her huge, cartoonish eyes. "I haven't been following the presidential races too much."

Bill Clinton was still in office when The West Wing, Aaron Sorkin's depiction of the White House during the fictional presidency of Josiah Bartlet (Dem, '99-'06) first aired, and although the final series ended more than a year ago, fans of the show still hold Janney in the kind of esteem usually reserved for dead rock stars. It was possible to love Bartlet for his nobility. You could love Toby for his brilliance and Josh for his sarcasm; it was even possible to love Sam and overlook the fact that he was played by Rob Lowe. But most of all we loved CJ, who ended a press briefing with the edict, "Set fire to the room. Do it now," and could find, in her facial expressions, shades of incredulity we didn't know existed. She responded to a reporter's complaint that she hadn't answered his question with, "How about that?" With the hauteur that only a 6ft tall woman in an Armani suit can muster, she'd conclude a meeting with, "Now if you'll excuse me, I have to cancel a photo op with a goat."

Janney has made three films since the show ended, but it's impossible not to look at her and feel that, this close to a presidential election, her rightful place is running down a corridor shouting "walk with me" and chasing up polling data. Her latest role might do something to unseat this. She plays Bren MacGuff, stepmother to the eponymous hero in Jason Reitman's film, Juno, and the kind of character that lesser films heap scorn upon. In Janney's hands, Bren's satirical elements - she works in a nail salon, has a daughter called Liberty Bell and sticks pictures of dogs on to something called a "vision board" - are tempered with real heart and steel. When Juno gets pregnant and is given a lecture in morals by a woman at the hospital, Bren sheds all her silliness and goes to war for her.

Juno was fun to make, she says, not least because it had so many strong, female parts, and "when I first read the script I said, 'Yes, I want to be a part of this.' " But Janney is still suffering from the anticlimax of life after The West Wing. "I miss it. I miss it a lot. I've never been with a group of people for that long. I mean, even my family I don't think I know as intimately as the people I worked with on West Wing. More than anything just to have a steady job, a sense of routine, is so lovely for an actor - you just don't get that very often. I really thrive in that condition. And then when I'm not acting I go, 'Ugh, I'm worthless. Why am I doing this? I quit. I'm gonna go back to Ohio and be a dog walker.' "

At the height of the show's popularity some of the news networks called to offer her work as a pundit. "I was offered all sorts of jobs by news organisations that should have known better," she says. "And I was like, 'That's so sweet. But you really don't want to hire me. I'm not good in that area.' " It's a shock to realise that she's talking about politics.

Janney has been in LA for more than a decade, but it isn't really her town. She lived for many years in New York and grew up in Dayton, Ohio, where both Martin Sheen and Rob Lowe are from, and considers herself midwestern by nature. "It's a generalisation, but people from Ohio tend to be salt-of-the-earth, friendly. That and coming from the theatre really grounded me when I came out to LA. And when all this hoopla started around West Wing, I was totally feet on the ground."

She is 48. For many years Janney's career consisted of trying to get casting directors to see past her height. "I'm not an easy fit." She studied at Kenyon College, Ohio, where Paul Newman was an alumnus and during her first year he returned to the college to direct a play, in which she was cast. Newman was impressed by Janney and told her to keep acting. Her mother was an actor, who'd studied at the American Academy of Dramatic Arts and once starred in a play with Tallulah Bankhead. "Now she's mortified; she would never get up on stage. She was a dancer, too," says Janney. "And then she met my father on a blind date and had the family and didn't do the acting any more. I don't know if she was any good, but I definitely know that I'm an actress because of her. Every character I do has a bit of my mum in her. She's like the doyenne of Dayton, Ohio."

After graduating, Janney moved to New York to study at the Neighbourhood Playhouse theatre. Every year the Playhouse gave a scholarship to one student to study at Rada for the summer, and in 1984 Janney won it. "I had the best time, I lived in the smallest apartment where the door opened and hit the bed and the sink was in the bedroom. And I'd go to the theatre every night with my student ID and pay four bucks to see a play. I felt very fancy, going to Rada. I thought, 'Now I am a real actor.' "

The problem with being a real actor, she discovered, was that it involved more luck than skill. All her early breaks were in the theatre. Janney did a play with Stanley Tucci and Marisa Tomei called Fat Men In Skirts, which Mike Nichols saw one night and sent her a fan note about. Years later he would remember it and cast her in Primary Colors. She appeared in a play written by Alan Ball called Five Women Wearing The Same Dress, and when Ball wrote the screenplay for American Beauty she was offered a part in it.

"I feel like if I'd had to audition I wouldn't have gotten those parts," she says. "I hate auditioning so much. I like people who go, 'You know what? Allison can do this.' "

It's a combination of nerves and experience, of years of being turned down for jobs. "I didn't think I was marketable for TV. I thought,' I just don't have the right look...' I tell ya, it's pretty awful. I'm a big girl, but I have a delicate constitution, emotionally. If I've been humiliated in some audition I just cry all the way home and think, 'Oh my God, I suck.' " She leans into the table and laughs.

It was the role in Primary Colors that led to her landing the job on The West Wing. Janney played Miss Walsh, a dowdy schoolteacher whom John Travolta's Bill Clinton-type character charms so thoroughly that she falls over while speaking to him. "I think it was the prat-fall I took that Aaron fell in love with. That's what I was always told."

The first thing she did when she won the role of CJ was panic. Her daily news intake at the time was the front page of the New York Times and the nightly TV news, and she knew nothing about politics. Janney grew up during the Watergate hearings and it put her off for life. "I think I was disillusioned. It seemed like everyone was lying and I didn't want to invest any time in it. The West Wing changed all that; I had to at least pretend to be interested in it."

For background she met with Dee Dee Myers, former press secretary for Bill Clinton, who told her that likability and a good sense of humour were the two best ways to do the job and bridge the gender gap. "She told me that, as press secretary, your relationships aren't determined by the boundaries of your job. It's by who likes you."

She had a "huge crush" on Sorkin and in the show's early days, dutifully researched all the issues in his scripts. She took tutorials from other members of the cast, chiefly Richard Schiff, who played Toby Ziegler, and Bradley Whitford, who played Josh and wrote two episodes in the final series. Martin Sheen meanwhile, "really believed he was the president. When I'd travel with him he'd walk down the airport corridor giving the peace sign to people when they called out 'Mr President!' "

There wasn't always time to do the homework, however, and more often than not, says Janney, "it was like, I have no idea what I'm saying; so I'd just say it."

Like what?

"Oh, God. I can't even... it would usually be about some Middle Eastern crisis and I wouldn't know if it was a good thing or a bad thing. And I'd ask the director, 'Is this good or is this bad?' And he'd say, 'It's bad.' And then I could..."

Adjust your expression?

"Exactly." She grins. "Listen, I'm an actor. I can make anyone believe anything. I don't have to know what the hell I'm talking about."

It's common for actors who spent years waiting for a break to talk, incredulously, about how close they came to quitting. For Janney, even with a hit show behind her, that feeling hasn't entirely evaporated. It's partly her disposition; she doesn't take herself too seriously and it obviously runs in the family - the first time she was nominated for an Emmy she rang her parents, "and my mum said, 'That's lovely, sweetie, but your dad and I are in the middle of fixing the septic tank.' "

But she is also less enamoured of her job than she once was. "I feel like I've lost my edge a little bit. I'm just tired of it. I don't care as much about certain things. As you get older you think I don't have to do that, or that." In one of her lowest periods, long before The West Wing, she took a load of aptitude tests at an institute in New York, to see what else she could do besides acting. "This woman sat me down and, well, at the time I thought she was so impressed with all the things I could do, and now I'm thinking I probably failed every one and this poor woman was trying to figure out what she could tell me. She told me that I'd make an excellent systems analyst." Janney leaves an artful pause. "I don't even really know what that is. I think it's someone who goes into a company if it's not working and makes it run smoothly again. But when I heard that I thought, 'You know what? I'm gonna stick with this acting thing.' "

These days, she thinks it might help if she strategised about her career a bit more. "I was really impressed by Lily Tomlin, who wrote Aaron Sorkin a letter and said, 'I really love your show and I want to be on it', and he wrote her a part. So why don't I write to Stephen Frears and say, 'I really love your work, I would love to be in a movie by you?' Does that work? I've never been that kind of person. I always wait in the corner for someone to say, 'Hey you, come 'ere.' "

When she thinks about doing something else, these days, it's to do with "painting and knitting, or making stained-glass windows, or water colours. My dream is to have a creativity barn in my back yard, which is full of musical instruments and every kind of paint and oils and paper and you can just go in and make something."

I tell her I hear that the craft movement is massive now. She bursts out laughing. "Oh my God, the craft movement?"

For the first time in her life, Janney is single. "I was in an eight-year relationship, a 10-year relationship and then a four-year relationship that ended last year." This was with the actor, Richard Jenik. "And now I'm single and don't see how I'm ever going to be able to date anybody, because..." she tapers off, in despair. "Because it's Hollywood and I'm a woman over 40 and it seems impossible. I think I'm done. That's the way I feel. I'm kind of depressed about that. A little bit."

Well maybe a nice Guardian reader will write in and woo her.

"Really?" She looks delighted. "Well maybe that would be nice. I would like to find somebody who was my age, or a little older; and who has a good relationship to the world, meaning, you know, financially independent. Someone who loves animals and they could have kids or not, doesn't matter." She smiles. "Someone who will just adore me and appreciate me and buy me some diamonds."

Well, she might have to look to the right-wing press for diamonds...

"...OK, maybe not diamonds. Other stones are OK."

"How about tofu?"

"I don't mind tofu."

Does he have to be an actor?

"No. I'd prefer not. So how is this going to work?"

Janney writes down her address, to which readers' letters will be forwarded on condition they don't use green or violet ink or contain multiple exclamation marks. (FYI, her favourite TV shows are 30 Rock and Curb Your Enthusiasm.)

She's thinking, this year, of starting her own production company. "Katharine Hepburn did that. That's how she got parts." And she'll be appearing on stage in LA in a musical version of the Dolly Parton film 9 To 5, in the Lily Tomlin role. Meanwhile, more than one Democratic candidate has asked her to campaign for them. Janney hasn't made up her mind who to back. "It's between Obama and Hillary, and I keep going back and forth for different reasons." She smiles and offers a rare, political judgment. "Just the fact that we're deciding between a woman and an African-American candidate is a nice a place to be." ·

· Juno opens on February 8.