Allison Janney has broken her foot. An ungainly lump of bandages swathed around her left leg contrasts comically with the rest of her elegant, understated outfit. The four-times Emmy-winning actress and star of The West Wing, one of America’s most lauded television series, is a little embarrassed to admit that she sustained the injury “being silly on a trampoline”. She has said that she wants to take risks now that The West Wing is over, but I doubt that hurling herself off a piece of gym equipment was what she had in mind.

But the temptation to go a little crazy must be there. The role of C. J. Cregg was an all-consuming undertaking that Janney inhabited for seven years with warmth, wit and a deliciously dry delivery. The trademark combination of big heart and a sharp tongue endeared her to viewers everywhere. The end of The West Wing has been as much a wrench for the actress as it has for fans. “Yes, it was very sad to let that go,” she says. “It didn’t hit until months later; these things never do because you’re used to taking a break. In some ways it was a relief, but also it was terrifying to think, oh shit, I’ve got to find another job and not wanting to, but wanting to crawl off and hide instead. Then part of you is excited about the possibilities – my God, I can do whatever I want.” It’s not for nothing that The West Wing was referred to by its cast as “a golden handcuff”. Janney talks about the novelty of being able to attend the family functions such as weddings and christenings that the shooting schedule previously made impossible.

West Wing fans, says Janney, are generally smart enough not to get her confused with the character she played. “We used to joke that Martin Sheen was the only one who got confused – he actually thought he was the President.” But the same can’t always be said of casting directors. Despite some astute career choices before and during The West Wing ( The Ice Storm, American Beauty, The Hours), it is the TV series that has dominated her work and the role of C. J. that has shaped casting directors’ perceptions. Now, she says, she is constantly looking for things that are slightly different from C. J., although she concedes: “I loved playing her, she’s a remarkable woman.”

With her low, smoky voice and self-deprecating humour, Janney is an obvious choice for comic roles. The postWest Wingparts so far have been the neurotic mum Prudy Pingleton in Hairspray; and now the Weimaraner-loving Bren MacGuff in Juno, the fiercely supportive step-mum of a pregnant teen with an unorthodox solution to her problem.

It’s not hard to see what appealed to Janney in Juno. The script, by the 29-year-old debut screenwriter Diablo Cody who has been nominated for a Bafta for her screenplay, is a showy display of zinging lines that crackle and snap like fireworks. But there’s an egalitarian generosity to the writing – while Ellen Page as Juno is the undisputed centre of the film, every character gets a share of the best gags. Janney’s obvious love of sharp-tongued wisecracks is rewarded with a handful of devastatingly funny putdowns. Her acidic tirade against a hapless ultrasound technician is one of the funniest moments of the film.

Janney says: “Diablo Cody’s script for Juno was very, very special. It struck me in the first two pages – her dialogue and rhythms. At first you wonder, is it believable? [[ But then I met her and yes, that’s pretty much how she talks.” How rare is it to find such a distinctive voice in a new writer? “It’s really rare. There are so many writers and storylines that are too familiar.”

Janney is quick to give credit to the director, Jason Reitman, as well. “Jason was really good at talking to actors and getting what he wanted without making the actor feel like they were bad. I worked with his father (Ivan Reitman) on Six Days and Seven Nights.He has a very different bedside manner – he’s a little intimidating.” So what does she look for in a director anyway? “Most of the time, I just want them to give me the part.”

The unshakeable confidence of her young colleagues in the film (Reitman is 30, Page a very self-assured 20) has led Janney to reflect on her own career. “I certainly didn’t have that confidence when I was that age. I was kind of a late bloomer. A lot of things happened to me later in life. I didn’t have much success as an actress in my twenties. Or my thirties. I was 37 when I got my first Broadway show. That’s when my career started.”

Janney grew up in Dayton, Ohio, the daughter of an actress and a real-estate agent. The Janneys were musical – though Janney’s contribution to family jam sessions was playing the snare drum. Music is still important, although she admits to being a technophobe who doesn’t know how to work an iPod and who still labours over compilation cassette tapes.

Rather than acting, the young Janney was drawn to figure skating. This career path ended after an horrific accident at a party which nearly claimed Janney’s life. “I went through a plate glass window. Someone stepped on the back of my dress and it started to fall off and I ran to get inside. I chose the wrong door to run through and the glass fell on top of me and guillotined my leg.”

After a lengthy recuperation, Janney attended Kenyon college in Ohio, fortuitously starting in the year that Paul Newman, a Kenyon alumnus, came to direct a play. “Everybody in the whole school auditioned. Even people who didn’t want to be in the play – they just wanted to meet Paul Newman.” Knowing that Newman was a fan of fast cars, Janney impressed him with a story that colourfully embellished her driving prowess. Newman and his wife, Joanne Woodward, encouraged the young Janney to pursue her career. “I think of them as sort of mentors,” says Janney. “Paul Newman once said he would do me a favour if I ever needed one. I still haven’t asked him yet. It’s one of those things I want to have in my back pocket.”

But, with the acting jobs elusive, Janney often considered quitting, even taking a career aptitude test. “They said I would be an excellent systems analyst. I don’t even know what that is, but I’m pretty sure I wouldn’t make a good one.”

Fortunately, a Broadway role was offered just at the crucial moment when Janney was about to give up for good. Since then her career has been a healthy mix of serious theatre and meaty film and TV roles. Even so, the insecurity of the job is something she finds draining. “I’m a little discouraging of young people who want to be actors. Do you really want to do it? It’s so hard.” The end of The West Wing was particularly tough because of the loss of routine. “It was the first time as an actor I had a steady job. I felt on top of the world, I really did. So that was a very hard transition.”

Home for Janney is in Los Angeles and is shared with a new puppy, a rescued blue-mottled Australian cattle dog. The key to staying sane in a place as crazy as LA is “exercise, the dog and some good friends”. She still gets to hang out with some of her West Wing buddies when their work permits.

So, for old times’ sake, could she be persuaded to put on CJ’s head once more and comment on politics? Is America ready for a female president? “Absolutely. I so believe the world more than ready for a female president – or an African-American president. I can’t believe we just had eight years of what we had. There has never been a time when we were more ready.” Juno is released nationwide on Feb 8

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