"I just have to love something so, so much in order to not be with my daughter all the time," says Jennifer Garner on the subject of Juno. This year's Little Miss Sunshine – similar tone, same film studio, same gargantuan, unexpected box office and critical success, including four Academy Award nominations – the film has already made more than $70m in the US, surpassing Sunshine's own against-all-odds haul.
Garner's role in the film is small – she is the mother hoping to adopt pregnant teenager Juno's child – but she seems unconcerned about such trivialities as minutes of screen time. "It is such a special little movie," she says with characteristic understatement. "I only had two weeks on it but waited a year for it to come together. And what a star Ellen Page [Juno] is." When photographers screamed at Garner to pose for them at the film's LA premiere, she insisted on being photographed with the 20-year-old Page, loudly reprimanding the snappers by jibing, "Ellen, this is the last time you'll be on a red carpet without everyone yelling your name!" No kidding. Page has already won a slew of awards for the role, not to mention that Oscar nomination.
Garner, who turns 36 in April, faces her own moments in the celebrity sun with a resolutely upbeat attitude, which can be ascribed to both a successful film career and to her family: she married Ben Affleck in June 2005, and the couple's daughter Violet is two. She remains close to the rest of her family – her mother, a retired English teacher, her father, a retired chemical engineer, and her two sisters, who both have families of their own.
Last year, Garner turned down what she tells me was a "big, exciting movie" to play Roxanne in Cyrano de Bergerac with Kevin Kline on Broadway. "When they asked me, I said, 'Yes, it's all I've ever wanted to do.' Growing up, my parents didn't have a lot of extra money but somehow we always had enough money for theatre. If we were in London all staying in one room in some horrible little bed and breakfast, we would stand in the half-price ticket line and see theatre – every night, not just sometimes. It was what we did as a family."
Cyrano was "a dream come true. Even before I did it, even mentioning it out loud made my stomach drop to my knees. Twelve years ago I auditioned with Kevin for something and that audition is part of why they thought of me. Isn't that crazy?"
Broadway critics are not often kind to TV stars (Garner made her name as the athletic, feisty spy Sydney Bristow in JJ Abrams' TV series Alias) but they lapped up this one. "The latest in a series of boldface film and television actresses to test their stage legs (including Julia Roberts and Claire Danes), Ms Garner seems by far the most comfortable," raved The New York Times.
No surprise to those who knew of Garner's back-up plan had Hollywood not come calling: "I probably would have gone back to college and taught acting or taught dramatic literature in some way. I could have lived in Atlanta and worked in the great theatres there. That would have made me feel really happy. That's what my goal was: to find a city that had good regional theatre, move there and be part of a company and have a family."
She is every bit as winning in person as she is on stage. When I mention I have been up all night with my vomiting toddler, she is like a concerned mother hen. When I see her again the next day – after she has endured a barrage of press interviews – she immediately asks if he's feeling better. She insists, like every parent on the planet, that she is quite different since Violet's arrival.
"I feel everything more deeply. I think that's pretty universal. You feel fear more deeply, happiness, joy, your child's sadness. And I just feel like a really lucky person who leads a pretty charmed life. I'm healthy, my family is healthy, I have a lot of people that I love and that I think care about me in a genuine way. I've been lucky in my jobs and in getting to do something that I love so much. I would say right now that I do feel like I kind of have it all. But I'm almost scared to even say it."
Garner is also rather brighter than many of her peers, a quality she shares with Affleck, though she is the first to admit that even she is a little intimidated by her husband's brain. "Sometimes I think, 'Oh gosh, I'd better read the paper so I can talk to my husband!'" she laughs. "I can hold my own with him, though. I hate to break it to him, but he likes to think, 'Can you believe how smart I am?' He'll be teasing me and I'll say, 'Yes, actually, I can. I'm astounded that you can keep up with me most of the time.' And I'll promise you one thing: he is not boring. You can't get him to go out somewhere, but at least you can have a good conversation with him. Just as long as you're willing not to leave the couch."
Garner says she struggles to find acting work that stimulates her grey matter. "So you have to grab it when you see it, definitely. But there's always something to learn from a movie, even say [her 2004 comedy] 13 Going on 30, which was so goofy. You learn what 13-year-olds are doing now and what it's like to work at a magazine, which I didn't know. There's always something to glean from it. And doing Cyrano meant just pumping my head full of the book and France in the 17th century, which I loved."
After filming The Kingdom, set in the aftermath of a terrorist attack on a US base in Saudi Arabia, Garner signed up for an online political science class. "The Kingdom was an education in and of itself. I realised it is a much more complicated world than what the movies show and I was compelled to find out more."
Nevertheless, she is ready for some lighter fare. She hopes to have a cameo in JJ Abrams' upcoming Star Trek film: "I just want to be a Klingon in the movie," she laughs. Another possible role could be that of political wife, since Affleck has long toyed with the idea of entering politics. "I think you have to be ready for anything when you're married to someone like Ben. But I do believe that after Gone Baby Gone [the film Affleck directed, unreleased in the UK due to similarities to the Madeleine McCann case], he's going to be stuck in directing for a long time."
Garner seems most content chatting about life at home. "I can make a beautiful loaf of wheat bread and I love to cook dinner. Last night I made grilled chicken with roasted sweet potatoes and broccoli from the farmers' market. I did a load of laundry, too," she adds, as if wanting to emphasise the normality of her life. Most of Garner's female friends are out of the business, with exceptions like Keri Russell – with whom she worked on another JJ Abrams series, Felicity – and Reese Witherspoon.
There is no prospect of Violet becoming an actor, not yet anyway. Affleck was a child actor and does not have fond memories of his experiences. "She absolutely can't work professionally until she's a grown-up," says Garner. Instead, she would like Violet to inherit Affleck's "curiosity" and her own "happiness. My mother has always had this mantra: 'Happiness is your own responsibility', and I really believe that to be true."
'Juno' opens on 8 February