By Christine Kearney

NEW YORK (Reuters) - Australian Isla Fisher was about to pack it in as an actress until her boyfriend, best known as Borat, said she should try comedy.

After 15 years acting in serious roles in two Australian soap operas, smaller parts in British and American television and movies, and one German horror film, she said Sacha Baron Cohen's suggestion was a revelation. And it worked.

Fisher, 33, is now appearing in her first lead role in a major Hollywood film. The romantic comedy, "Confessions of a Shopaholic," was released in the United States on Friday.

"I remember when I was auditioning for all these dramatic roles and getting rejected and really disheartened," she said in an interview. "And I just thought 'That's it. I am done.' And he (Baron Cohen) said to me, 'You should be doing comedy, you are so funny.' And I said, 'What?'"

"And I thought 'Well, if he is the funniest man in the world and he thinks I am funny, then maybe I should give it a go.'"

That was just before her breakthrough comic role in a supporting part in the 2005 film "Wedding Crashers," which helped her land the starring role in "Shopaholic."

Based on the best-selling series of "Shopaholic" books, the film has been compared to "The Devil Wears Prada" and "Legally Blonde," although the petite, red-haired Fisher sees it as more slapstick, along the lines of Lucille Ball's work.

In early reviews, she earned high praise for her performance alongside an experienced supporting cast, including John Goodman, Kristin Scott Thomas and Joan Cusack.

PRESSURE WAS ON

Fisher said she felt a lot of pressure adapting the character of Rebecca Bloomwood, created by British novelist Sophie Kinsella, for movie audiences and playing the lead in a big Hollywood production.

But while her character is a single, debt-ridden, fashion-obsessed shopaholic, Fisher's life is about breastfeeding her baby, dressing down and shopping in bookstores instead of fashion boutiques.

"Because I am a mum, my uniform is jeans and trainers and T-shirts," she said. "I was raised as a tomboy with boys and I never really feel like myself when I am really dolled up at premieres and showbiz events."

Fisher, who co-wrote two teen novels with her mother and starred in the Australian soap opera "Home and Away," was born in Oman but grew up in Australia.

She attended drama school in France and landed smaller parts while embarking on a relationship with Baron Cohen, with whom she lives in Los Angeles. They have a year-old daughter, Olive.

Fisher credits her success to the resurgence of her silly side.

"Once I stayed true to the fact that I have always been the one to make a fool out of myself, then all the doors opened for me," she said.

(Editing by Bob Tourtellotte and Stacey Joyce)