When she was a young girl, climbing trees in the wilds of Australia to escape a feral pack of brothers who called her Slug Face and worse, Isla Fisher dreamt that one day she would live in a big house in the Hollywood Hills. Only she thought she would be the maid. Proving that in Los Angeles dreams do come true, just rarely in the form expected, Isla recently had to face the drama of being mistress of her own glass-and-steel Hollywood Hills domain as the stars came calling for her western-themed barbecue.

There is Jennifer Aniston, lurking around the grill without absorbing a single calorie (how does she do that?); Ben Stiller and Dustin Hoffman playing the fool with neighbourhood kids; Ellen DeGeneres and her spouse, Portia de Rossi, fondly demonstrating once again why Hugh Grant spluttered into sniggers when asked by a hapless interviewer if he was wooing Rossi; and Courteney Cox and David Arquette bemoaning property prices.

And there is Fisher, one half of Hollywood’s latest power couple, dressed down in a red check shirt and faded jeans, introducing guests to the star of the show, baby Olive, just one year old. And the daddy, Sacha Baron Cohen, in a cowboy hat and red bandanna, a marked shift from his other favoured Hollywood party gear, a fully teated cow construction (“It was the only costume left in the shop in extra-long,” according to a still-amused Isla), or the overexposed and overexposing Borat bathing suit, which is “away in the wash somewhere, I think . . . I hope”.

It’s not as if LA is truly home to Fisher. Born in Oman to Scottish parents working for the United Nations and raised in Western Australia, she always feels somewhere between London and Oz. But she has learnt to grasp the moments. And this was a rare moment indeed, with Baron Cohen snatching a breath between the guerilla-style filming of his latest movie about an outrageously camp fashion fanatic called Bruno, and Fisher having wrapped filming in New York of her even more colourfully dressed comedy, Confessions of a Shopaholic.

Now, a few weeks later, nestling into a big sofa at the Four Seasons Hotel in Beverly Hills promoting Confessions, her first big comedy since she broke through as a flame-haired nymphet in Wedding Crashers, Fisher is starting to feel a little more comfortable in the city of second-hand angels. “There are photographers outside the house, but I can pull a baseball cap over my hair and slip out without being noticed. It’s not so easy when I am with Him — the size difference is absurd. My secret — and that of most actors in LA, the 98% of them you don’t see in the magazines every week — is sticking with a small group of friends who I trust, keeping it low-key and not behaving like I do in movies.”

It’s fair to say her performance as Becky Bloomwood in Confessions is standout comedy. It’s a knowing, sexy, sassy addressing of what all girls know they have in them somewhere, in however great or small a measure: a potentially disastrous relationship between their wardrobe, their credit card and the shops. Oh, and it has the funniest dancefloor turn since John Travolta and Uma Thurman’s two-fingered salute in Pulp Fiction.

“These days I am very much a girlie girl,” says Fisher, 32. “I used to be a tomboy, always outdoors. I grew up with a lot of brothers, so this is all different for me. Now I have a great group of girlfriends and no men in the gang. Don’t know why — I guess they don’t fit into the girlie dinners and teas and going out in a posse.”

So what does Isla’s posse, like, do? She is studying Judaism, better to understand her man’s bloodline, and declares she wants to become a mohel, a Jewish man who circumcises babies, and use her religious status to scare the wits out of boys. “So I go out with the girls and the knife, and when I am in interviews, I shall just sit there and sharpen it . . .” She starts howling at the absurdity of her escalating comic vision. “We lurk inside my house waiting for the UPS guy, and we all leap on him and . . . no, no, joking.”

This is a rare flash of the unguarded Isla, the 5ft 2in fireball who exploded from Methodist Ladies’ College, Perth, to play Shannon Reed in the Oz soap Home and Away. This is the caffeinated tabloid queen who once told reporters how she used a hapless young thing to lose her virginity only to dump him the next day, how she wanted to burn down the house of one ex and strangle the pet of another, and who fessed up to a fantasy about Mr Bean. “It was always said with a wink, but that wink never came across. So I have had to shut myself up, because I have more to protect now. Some subjects are off limits, I’m afraid,” she says, and for a moment seems genuinely embarrassed.

Most obviously these subjects include Baron Cohen, whom she namechecks only once, as she recalls how they turned tail from a cinema that was showing Slumdog Millionaire. “They only had two seats apart and, as it was our seventh anniversary, it didn’t feel right to sit away from each other, so we just walked around in the New York snow.”

It was a rare night away from her infant, glimpsed during the interview through frequent BlackBerry checks with the babysitter, but never named. “It can be tough because motherhood is one of my most favourite things ever,” she says. One senses the tension because she is a gregarious, full-throated laugher, a woman who loves to entertain, maybe even needs to please people around her. There was certainly a hunger that drove her to trade Home and Away for acting school in Paris (where she qualified as a clown) and took her to Hollywood.

Today, the redhead describes herself as a comic actor, but she never had the confidence to commit to full-time laughter-making before she hooked up with Baron Cohen. They reportedly met at a party in Sydney, and he proposed over milk and cookies in the garden of the Chateau Marmont hotel in LA in 2004. And since then her star has been in the ascendant, from Wedding Crashers and on to Confessions, the film version of the first book in Sophie Kinsella’s globe-spanning series.

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