Isla Fisher: Confessions of a seriously funny girl

She may not be as outrageous as her husband, Sacha Baron Cohen, but Isla Fisher is no shrinking violet. OK, she hasn't undertaken same-sex nude wrestling, crashed the Oscars' red carpet or posed in Cannes in embarrassing beach-wear, like the British star of The Dictator. But that doesn't stop the Australian actress chattering away faster than a pair of wind-up novelty teeth; so much so, as she told one US chat-show host recently, she would frequently be approached at parties for cocaine, her friends telling her, "We thought you were high as a kite most of the time."

Now 36, Fisher swiftly explains that she's never even seen cocaine, let alone tried it. The pert Perth red-head has a butter-wouldn't-melt smile that (almost) convinces you this might be true. It's why, she says, she likes to play crazy when she can – from the psycho girl in Wedding Crashers to the "cokehead whore" she recently took on in the comedy Bachelorette. "I don't take drugs, I'm a mum, I don't really drink any more, so to go on to the other side and do something completely different to who I am is really fun."

The pale-skinned Fisher, who today is dressed in a strapless, low-cut black dress, has Celtic roots – she was born in Oman to Scottish parents – but spent the majority of her upbringing Down Under, after her family moved to Perth, which has given her a typically sunny Antipodean outlook. She's far more open than her husband, whom she met at a party in Sydney in 2004, married six years later, and settled with in England. While he shuns interviews unless he's in character and maintains a certain mystique, she is quite the opposite, as candid as she is colourful.

Still, she paints an interesting picture of life with Baron Cohen. Away from comedy, they're both foodies, she says, and, when they're back visiting her homeland, love to swim and sail. But how is it to live with an actor who is known for immersing himself in characters for months? "I think it's more the facial hair that's disconcerting," she quips, referring to his hirsute role in The Dictator. "The handlebar moustache, the big beard… it's the look of the character [that can get on your nerves]." While she reports that he did wake her up at 5am and shout "Death to the West!" during the film's press tour, has he ever struggled to come out of character? "No, no, no," she says, hurriedly. "He's not insane!"

Famously, Fisher converted to Judaism when she married Baron Cohen, though she claims she has no need to temper her on-screen behaviour to fit with her new religion. "My husband's Jewish and the things he does are pretty out there! I don't see a connection between the two." She also refuses to go naked for the camera. "I don't know. I wouldn't feel comfortable doing that." Indeed, with two young children around – Olive, five, and Elula, two – much of their household's output has become quite family-friendly, with films like Hugo and Confessions of a Shopaholic. "See how conservative we are?" she grins.

It's doubtless why her latest film is an animated feature, Rise of the Guardians, a Yuletide-set 3D DreamWorks effort that sees a group including Santa Claus, the Easter Bunny and the Sandman team up to fight against the Boogeyman. "It's like an animated Avengers," laughs Fisher, who plays the Tooth Fairy, a half-human, half-hummingbird creature that sounds rather like her. "My character is very hyperactive. She's like a type-A personality – she's very turned on by teeth! There's a lot of humour to my character. I've milked it dry. I made every joke I possibly could about being excited over molars."

Her love of a good gag, she says, comes from her childhood growing up with four brothers. Their father worked for the UN, which meant a different school every year – so a self-deprecating sense of humour helped her make new friends. "Also, when I was growing up in Australia, there were really tall model-y girls and I was a short red-head, and there's only so much a push-up bra can do – you've got to get funny! You've got to take out the competition with some jokes – so that was my pulling technique!" She grimaces. "I'm going to regret that quote!"

Growing up without watching films – her parents only took her to see The Dark Crystal and E.T. – Fisher was turned on to acting by watching her mother play in an amateur production of Twelfth Night. "We had to go to bed really early while she was prancing about singing songs, and I wanted to be a part of it." By the age of nine, she was appearing in commercials, before making inroads in television – notably in that perennial Aussie soap Home and Away. Her Hollywood break came in a supporting role in the live-action 2002 film of classic cartoon Scooby-Doo, before Wedding Crashers sent her global.

While Fisher has mainly plied her trade in comedy – bar 2007's crime film The Lookout, which saw her use all her charms as the seductress Luvlee – she's rarely dipped into drama. But next year will see her in The Great Gatsby, Baz Luhrmann's 3D spectacular take on F Scott Fitzgerald's novel, playing social climber Myrtle Wilson. It was, she says, a "surreal" experience to work for Luhrmann. "He's my dream director. I've only ever had a short list of people I've wanted to work with, and he was at the top of it. I honestly couldn't stop smiling the whole time."

She's also just finished Now You See Me, a thriller about a group of illusionists that pulls off bank heists, alongside Mark Ruffalo and Morgan Freeman. Then there's her love of writing; in the past she's published two "trashy chick-lit" novels – including Seduced by Fame, about a waitress who wins a role in a soap opera. She wants to do more, has even knocked out two scripts, "but it's hard, with having a family and trying to keep my acting career ticking along". No wonder Baron Cohen is bonkers over this lady.

'Rise of the Guardians' opens on 30 November