The scariest animal? Definitely the parrot!’

Matt Damon hesitated before signing on to star in We Bought A Zoo, a film featuring both children and animals. But it wasn't WC Fields' quip 'never work with children or animals' that troubled him – for the dashing star of the Bourne films, it was the prospect of appearing too sentimental.

'I was mainly concerned the movie was going to be cheesy. But Cameron has a tendency to be really good at working in the vicinity of cheesiness without being cheesy,' he says, referring to Zoo director Cameron Crowe, who also directed Jerry Maguire.

Damon, 41, doesn't mind admitting how much his attitude to life has shifted since 1998 when the exuberant and talented newcomer shared his best screenplay Oscar with childhood friend Ben Affleck for Good Will Hunting. He went on to date a stream of Hollywood stars but today he is a contented family man and husband to Luciana Barroso, the unassuming bartender whom he fell in love with at first sight eight years ago.

'Ten years ago, I wouldn't have been able to play this part. I just wouldn't have understood how to. But now is the right time to do it because of being a husband and a father.'

Chatting in a New York hotel room overlooking Central Park, about 10 minutes from the home he shares with his family, he struggles to find the right words to describe how his emotions have changed since becoming an instant dad to his wife's daughter Alexia, now 12. The couple have since gone on to become proud parents of Isabella, five, Gia, three, and 16-month-old Stella.

'I jumped in at the deep end as Lucy had a four-year-old when I met her so that was really my first experience of having a child in my life.

'It's a silly way to describe it but I always think of the story How The Grinch Stole Christmas. At the end, the Grinch's heart grows three sizes – that was my experience of parenting. So with that came more access to my emotions, I think,' he says, scratching his newly-shaved head, fresh from filming Neill Blomkamp's Elysium in Mexico.

Though Damon's We Bought A Zoo co-star Scarlett Johansson is best known for her glamorous roles, she welcomed the chance to appear without make-up as a zookeeper.

'I'd look ridiculous if I was shovelling muck wearing false eyelashes,' laughs Johansson. 'How silly it would be, feeding animals wearing lip gloss.'

She admits peering into the enclosures of her four-legged co-stars on the set of We Bought A Zoo elicited moments of empathy. 'It is a bit like being in a zoo,' she says of the escalating levels of curiosity about her private life, a fascination that began back in 2003 when she officially turned from child actress to adult movie star with roles in Girl With A Pearl Earring and Lost In Translation.

'There is certainly a freak show aspect for someone who is recognisable. But it comes with the territory, and over time you just learn to cope with it,' says Johansson, 27.

'I feel incredibly fortunate to do what I love to do. But I know so many actors who can't get a job, so to complain about it would be a little silly, even if it is annoying.'

The actress is currently dating advertising executive Nate Naylor, and is enjoying the relative anonymity that comes from being with someone who isn't in Hollywood. Her role as a zookeeper meant dealing with some very unglamorous situations, including chopping up baby birds to feed to the reptiles.

'I think the most surprising thing for me was just going into the food locker. The food locker of a zoo is vile and disgusting, quite gross. When you prepare food for a turkey vulture, for instance, you take a frozen baby chick out of a bucket of frozen baby chicks, and chop it up like a julienne of vegetables, and mix that all up,' she says. 'But if you're caring for an animal, you can't be precious about it. You just have to get down and dirty.

'I was much more scared being around the parrot than I was around any of the big cats. I don't know why. I just don't like parrots and I'm not a big fan of the rodents, either. The monkey was cool, although mostly I'd rather be at home sitting on the couch with my chihuahua. My life is not always a press junket. Most of the time when I'm not working I live a relatively normal life. I don't leave hotels and get mobbed all the time,' she says.

It's little surprise to learn how she bonded instantly with her 13-year-old co-star, Elle Fanning, who is experiencing the same giddy career rise as Johansson herself did as a youngster when she starred in The Horse Whisperer and Home Alone 3.

'I love her so much,' says Johansson. 'She's got it pretty " gured out. I remember working when I was her age, so it was interesting to witness how she goes about it. She's just incredibly professional. She's been doing it for so long and, hopefully, she'll be doing it when she's my age. I think we started roughly about the same age, seven or eight. It was kind of a full circle for me to see her at work.'

Reflecting on Fanning's career causes Johansson to think about the changes in her own life.

'Ten years ago, I was 17. It's very different when you're a teenager. Since then I've grown a little bit more tolerant. I'm pretty impatient and slightly controlling, but maybe I'm more aware of that now. I think I've learned that I don't have to completely lose myself in order to lose myself in a part. I think I'm willing to take risks, for better or worse. And I appreciate that quality in other people as well.'

We Bought A Zoo is on general release.