After spending nearly half his life in show biz, Jonathan Lipnicki was thrilled to film a movie in which he wasn't the shortest actor on the set. The nine-year-old's co-star in Stuart Little, which opened yesterday, was actually a three-inch-tall, computer-generated mouse. But Lipnicki isn't going to quibble about details.

"Yeah, he was smaller than me," Lipnicki grins, revealing a gap where his two front teeth used to be.

Lipnicki has just wolfed down a grilled-cheese sandwich and is squirming in his chair at a Midtown hotel, fidgeting with repressed energy. The kid star, who charmed audiences in Jerry Maguire, says that acting opposite an imaginary figure - the epony-mouse in Rob Minkoff's adaptation of E. B. White's beloved children's classic - was "weird but fun."

"We had to hold balls in our hand and pretend they were the mouse - there were silver balls and white balls, all different colours," he says.

"I saw some little stuffed dolls of Stuart before we filmed, but they didn't look anything like him. He's a mouse that looks kind of human, and I thought he was going to be just a regular mouse dressed up."

Lipnicki's life to date has been anything but regular. He hit it big his first time out, helping make Jerry Maguire a blockbuster in 1996 (he was five and starred opposite Tom Cruise) and winning the National Broadcast Film Critics Association's award for Best Child Performance of the Year.

He went on to voice the character Baby Tiger in 1998's Dr. Dolittle, starring Eddie Murphy. He also had the starring role in the CBS series Meego and was a series regular on The Jeff Foxworthy Show, in which he forged a friendship with his co-star - the eleven-year-old Sixth Sense star Haley Joel Osment.

Lipnicki's screenwriter mother, Rhonda - who doubles as manager for him and his twelve-year-old actress sister, Alexis - says she and husband Joe, an accountant, "actively pursue normality." But she admits life for the Los Angeles-based Lipnicki's changed after Jerry Maguire. "It surprised us," she says. "Jonathan just became so recognizable. We'd go out and he kept saying, 'How come these people know my name?'"

With his spiky, white-blond hair and steel-rimmed spectacles, the cute-as-a-button kid with the burgeoning big-screen career sure stands out in a crowd. And how many nine-year-olds have ambitions of adding a little gold statuette to their toy box? "I think I will try and get an Oscar," Lipnicki muses, sipping his lemonade through a straw. "I just want to get one to beat the record in the Guinness Book. If I get one for Stuart Little, I'll be one year younger, because the record is held by Shirley Temple, and she was ten." It's not beyond the realm of possibility. Lipnicki's turn as George - a young boy whose eccentric but kind-hearted parents (Geena Davis and Hugh Laurie) return from the orphanage with a mouse as his new brother - has critics raving.

Lipnicki was prompted to get into acting by his sister, who was an extra in Jerry Maguire and also features in Lipnicki's next film, The Little Vampire, due for release next year. Alexis had joined a drama class, and her five-year-old brother begged his mother to let him go, too. "She was having a lot of fun," he says, "I thought it would be cool."

So, acting was added to an already full roster of extracurricular activities. As well as being an avid reader - he'd love to play the title character in the Harry Potter series - Lipnicki is also sports-crazy and a bit of a daredevil. He proudly displays the stitches on his thumb, the result of his latest skateboarding injury, and reels off a litany of minor accidents that have befallen him as he pursues his various hobbies - snowboarding, baseball, basketball, kickboxing, boxing and Brazilian jiujitsu.

Somewhere along the line, he's also going to have to fit in medical studies, as he plans to follow his uncle into the chiropractic business, explaining with a nine-year-old's logic: "I've got bad handwriting, and all doctors do, so I thought I'd be a good doctor."

But his mother says Lipnicki told her he plans to keep acting "until I need a walker."

"He told me he also wants to direct," she says, with a smile. "I have to believe him, because everything he sets his mind to he does."