She may not be much taller than a plant pot and her face may give her the illusion of not being long out of her teens, but don't be fooled: Isla Fisher is as tough and smart as they come.

Having ploughed her furrow in the soap opera field like many an Australian actress before her (she is a Home And Away alumna), Isla Fisher, as even she concedes, is enjoying one of the most fruitful periods of her life.

A burgeoning career, an engagement to one of Britain's most successful comedic exports, Sacha Baron Cohen (the man behind Borat and Ali G) and a four-month-old daughter in the shape of little Olive - it is no wonder that Fisher declares herself to be "the happiest I've ever been in my life".

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The long and short of it: Isla Fisher with Sacha Baron Cohen

The first time I met her, she was 23, had recently appeared in panto and was dating a certain Mr. Darren Day (yes, that one).

But now, at 32, she is working in Hollywood and engaged to a man that the Hollywood Bible, Entertainment Weekly, has listed as one of their Top 50 Smartest People in Hollywood.

From the ridiculous to the sublime in nine short years.

If she weren't so likeable it would be terribly easy to, well, really not like her.

"When I did my first audition for the movie Scooby Doo, they flew me to America first class," she remembers.

"At the time, I didn't have any money and then I was flown to LA and put up at the Mondrian Hotel. It was like landing on Mars!"

Since then, Fisher has adapted to Martian life with ease. After securing her role in Scooby Doo opposite Sarah Michelle Gellar and further minor roles including one in I Heart Huckabees with Dustin Hoffman, Fisher suddenly made everyone take notice three years ago in the Owen Wilson-Vince Vaughn comedy, Wedding Crashers.

Her show-stealing turn as the psychotic, hairswishing nymphomaniac to Vaughn's terrified suitor won her countless plaudits, and helped spark a definite upsurge in her career.

Her latest project sees her as one of the leads in the romantic comedy Definitely, Maybe.

Produced by the team behind Love Actually and the Bridget Jones movies (and, coincidentally, Ali G Indahouse), Definitely, Maybe tells the story of political consultant Will (played by Ryan Reynolds of Smokin' Aces fame), bringing up his ten-year-old daughter Maya (Little Miss Sunshine's Abigail Breslin) in Manhattan.

Currently going through a divorce, Will is encouraged by his daughter to replay the story of how he met and married Maya's mother, although he refuses to tell her who her mother actually is.

Maya - and the audience - have to decide whether he eventually wed Emily (Elizabeth Banks), his first love; independent-minded journalist Summer (Rachel Weisz); or his best friend, the apolitical April (played by Fisher) and whether he will ever be able to find love again.

The film is lightweight fun; ideal fare for its pre-Valentine's Day release, and one which Fisher enjoyed making because, "April is . . . sassy and feisty and she says what she thinks.

"Will and April have a very Spencer Tracy/ Katharine Hepburn- esque relationship. It's very combative, but there's a lot of sexual tension and love between them.

"I think some of the slick, Hollywood romantic comedies where you know in the first act who's going to end up with who aren't realistic, but this is definitely a realistic story where all the women are just normal people who are flawed.

"Would my past relationships have made brilliant romantic comedies?" She laughs.

"Comedies, maybe."

Despite romantic entanglements which have included the aforementioned Day and banking heir Anthony de Rothschild, Fisher has been happily ensconced with Sacha Baron Cohen for the past six years.

They make an interesting pair: he the 6ft 3in perpetrator of crimes against swimwear thanks to the lime-green thong he sported in Borat, and she, the 5ft 2in self-confessed "eccentric".

While they initially enjoyed a certain amount of anonymity in Hollywood, the success of Borat coupled with Fisher's increased profile have meant, she admits, that, "we're starting to have our pictures taken a little more often now, probably because Sacha's so tall while I'm so small, I'm like a circus freak!

"But I'm not really comfortable talking about my private life and Sacha and I are probably very similar in that we hide our feelings behind humour."

She's not kidding. While many celebrities profess to a certain discomfort in giving interviews, Baron Cohen has taken that idea one step further by giving interviews solely in character.

In a rare interview speaking as himself two years ago he admitted that, " essentially I am a private person and to reconcile that with being famous is a hard thing to do."

Certainly, it is a need for privacy which has rubbed off on Fisher too and which has only served to make the couple yet more intriguing - even more so, now that their friends out in LA include the likes of Courteney Cox, her husband David Arquette, Jennifer Aniston and Naomi Watts.

In an age where celebrities dish the dirt on themselves with alacrity, such circumspection is refreshing.

Even the couple's four-month-old baby, Olive, who has inevitably been dubbed Baby Borat by the tabloid press, is more or less out of bounds.

"You know, motherhood is my favourite topic in my personal life and I won't shut up about it, but it's not something I want to discuss publicly just because of the amount of attention it draws to a small person who didn't choose to be exposed," says Fisher.

"But I think that all experiences definitely change your perspective on life, particularly motherhood."

The couple got engaged back in 2004, although have yet to set a date for their wedding ("I'm thinking of not inviting Owen and Vince so I can see whether they can crash our wedding"), but Fisher has already converted to Judaism in keeping with Baron Cohen, a devout member of the Jewish faith.

"I love religions and find them fascinating and I find Judaism very beautiful," she says. "It's enriched my life enormously."

Certainly, little Olive is going to have to get her passport at the ready in order to keep track of her parents' relatives, as both Fisher and Baron Cohen have an eclectic heritage.

Sacha Baron Cohen's father Gerald, owner of the House of Baron clothing shop in London's Piccadilly, originally hailed from Wales. Sacha's grandfather was orn in Pontypridd.

Baron Cohen's mother, Daniella, as born in Israel, where her own mother - a German ballet dancer - currently resides.

Isla Fisher's parentage is no less colourful. Although now divorced (Fisher's mother lives in Greece while her father resides in Germany), her Scottish parents were living in Muscat, Oman, when Isla was born, due to her father being stationed there for his work for the United Nations.

Shortly after Isla's birth, she and her family (she has two brothers and two stepbrothers) relocated to Perth, Australia - an upbringing he has described as: "great and very outdoorsy... I don't know if I'd have had those same opportunities if I lived in Sydney or Melbourne."

Isla's parents divorced when she was nine - an event which when I interviewed her all those years ago, had a distinct impact on her life.

"You can't underestimate how traumatic divorce is for the children," she said.

"When your parents divorce, it makes you grow up fast. I'd urge parents to strongly consider working things out. I'd work things out and I'd definitely stay put," she said adamantly.

"Especially if there were babies involved."

As is the case with many children of divorced parents, Isla was a definite over-achiever, reading "frightening amounts of books" and developing a passion for acting as a means of "escaping, of being someone else and getting away entirely".

By the age of 16, she had landed a role in the Australian series, Paradise Beach and as the show was set in Queensland, a six-hour plane journey from Perth, Isla left home while a mere teenager, in order to pursue her career.

Paradise Beach, Fisher moved on to Home And Away where for three years she played Shannon Reed - the pretty yet confused Summer Bay resident who had affairs a go-go.

During her time on the soap, she also managed to have two children's books published - a talent for writing which she no doubt acquired from her mother, also a children's book author.

"Between that whole period of 16 to 21, I was a real workaholic . . . and I think a lot of that was down to wanting to escape."

After leaving Home And Away, Fisher relocated to Paris, where she enrolled at an acting school run by the famed mime artist and physical theatre actor, Jacques Lecoq.

She could barely speak French ("I relieved myself of the worry of looking an idiot by telling myself it was the language barrier") but graduated from Lecoq's school nonetheless and moved to London shortly afterwards where she dated - and was briefly engaged to - Darren Day.

They met while starring in a production of Summer Holiday and got engaged in December 1998, not long after his split from Coronation Street actress, Tracy Shaw.

The tabloids, naturally, gobbled up the story, not least because Day had already been dubbed a "love rat" for his serial romancing.

He and Fisher split up the following July, got back together again and then split up permanently in summer 2000 with Fisher instigating the final break.

When they split, Day admitted he was devastated and yet even then, meeting her as a mere 23 year old, something about Fisher suggested an older, more circumspect head on young shoulders.

Now that she is settled following the birth of her baby daughter, the hardworking, over-achieving days of her formative years are still not quite behind her.

She has already sold two comedy scripts to major Hollywood studios, has voiced one of the characters for the forthcoming Dr Seuss movie, Horton Hears A Who!, and is due to start filming Confessions Of A Shopaholic (based on Sophie Kinsella's bestseller), as the lead character, Rebecca Bloomwood.

"Although I'll be focusing on colouring in books for the next few weeks!

"I don't want to put the pressure on myself to have to race back to something."

So, did Fisher ever dream she'd get this far?

"Oh, I always thought I'd be living in Hollywood, in a seven-bedroom house with a massive swimming pool," she grins.

"It's just that when I imagined it, I always thought I'd be the maid."

• DEFINITELY, MAYBE is released in cinemas-nationwide from