Jennifer Garner: Actress with the ex factor

There's nothing like a little healthy sibling rivalry to spur you on to greatness, according to Jennifer Garner. As one of Hollywood's top actresses, Garner, 37, enjoys all the trappings of fame including a new $17m mansion, a lucrative celebrity endorsement with Neutrogena and two children with her husband, Ben Affleck.

Yet remarkably, it's her big sister, Melissa, 40, whom she credits for her success.

"If I'm totally honest, I wouldn't be where I am today if it wasn't for the fact that I have this bigger-than-life, incredible older sister," says the actress, who also has a younger sister, Susannah, 34. "She's beautiful, and she was valedictorian, got a 1600 on her SATs [the top score on the US high school exams] and was the head majorette. I was just the middle kid, kind of looking for attention. So that's what drove me, I think, to do things she wasn't doing."

The daughter of a chemical engineer father and English teacher mother, she was a chemistry major at Denison University before she surprised her family by suddenly switching to a drama degree. "I just realized that I liked drama more than science. At the time, I planned a career in the theatre; I didn't even consider film or TV," says Garner, who went on to study at the National Theater Institute in Waterford, Connecticut, before moving to New York where she earned $150 a week as a theatre understudy.

"I certainly never expected to be in front of a camera one day of my life," she says. "Even when I was studying acting, I wouldn't even go into the film department of my school. I was only about the stage. I was a total snob. And then when I moved to New York and was trying to get a job on stage, and was broke and got offered a TV movie, I was like, 'Hot dog, put me in front of that camera. Let's do it!' And now that it has continued to evolve, I still love my job.

"Also, just looking at it as a working mum, there are a lot of great things about what I do. I work really intensely for a while but in a way and in a place where people are very respectful of me and of my kids. Also, if I need to take a break and feed a baby, they let me, which isn't always the case for women in the corporate world. I have help and I have the support that I need. I have a great partner in raising kids, too, and he's super involved and there with me. On top of that, I just got to take nine months off from work. I mean, who gets to do that? So it's so far, so good."

After winning roles in the TV series Time of Your Life and Felicity, she got her first major film break starring as Ashton Kutcher's girlfriend in the forgettable Dude, Where's My Car? But it was her role as Sydney Bristow in the spy drama Alias which brought her to Hollywood's attention: Garner eventually moved to Los Angeles to capitalise on film opportunities with roles in Catch Me If You Can, 13 Going on 30, The Kingdom and Juno.

Currently starring opposite Matthew McConaughey in the romantic comedy Ghosts of Girlfriends Past, she plays a single career girl, nursing a broken heart.

In real life she has no such romantic woes. She is happily married to Ben Affleck, with whom she has two daughters – Violet, three, and Seraphina, three months. Married for four years, she clearly never viewed her husband as a tough catch, despite the fact that his former romantic baggage includes Gwyneth Paltrow and Jennifer Lopez. If other women might have been more wary, instead she reveals: "I've never had a problem resisting somebody that I knew was going to break my heart. Going after the "bad guy" has not been a real issue for me. One thing that I liked about this role was that it sounded like a version of conversations I've had with tons of girlfriends where it's just: 'stay away from him!'"

Having previously starred alongside her future husband twice within the space of two years in Pearl Harbor and Daredevil, she's adamant that she won't be signing up any time soon to work with Affleck, saying today:. "No, no, no. It doesn't work," she says, refraining from any overt reference to her husband's personal foray into that realm five years earlier with then-fiancée Jennifer Lopez, together co-starring in the poorly-received films Gigli and Jersey Girl. "It would just feel so incredibly weird. I couldn't do a scene with him for all the tea in China."

Garner is that rare celebrity who is unwilling to milk her marriage and private life in exchange for column inches: "The rule with marriage is the less you talk about it the better. Same with my kids. I believe that they – even though the rest of the world doesn't necessarily believe this – have a right to their own privacy. Ben can take it. He doesn't care, but as far as the intricacies of our relationship and family, that's something that automatically makes me go 'Ewwww... '"

In Ghosts of Girlfriends Past, McConaughey plays a womanising glamour photographer forced to confront the women whose hearts he's broken – Garner's character being one of them.

Ask the actress if there's any heartbroken men from her own past to whom she might wish to apologise, and she's quite firm. "Only if you need to apologise for growing up and making mistakes and learning just basic things," says Garner, who was 28 when she wed fellow actor Scott Foley, divorcing two-and-a-half years later.

If Garner has suffered any career disappointment, then it was 12 years ago when she worked with Woody Allen on Deconstructing Harry. "I seriously worked on the film for maybe two weeks – and then I was not in the movie. I was in awe the day that I was asked, so of course I would love to work with him again – although I don't know if he would remember me since he cut me out of the movie."

Next up, she stars in The Invention of Lying, due out later this year, a comedy written, directed and starring Ricky Gervais. "Ricky is very specific about things, although the worst thing about him as a director is that if something is funny at all – and it was very improvisational – he breaks and laughs and ruins your take. I would say to him, 'It's your movie, but can you just leave? Maybe just go into another room?' But he knew exactly what he wanted."

' Ghosts of Girlfriends Past' is in cinemas now