DEFINING WHERE `HARRIET' ENDS, MICHELLE TRACHTENBERG BEGINS.

Byline: Barry Koltow Orange County Register

Michelle Trachtenberg just can't help herself.

In midinterview to publicize her film debut in ``Harriet the Spy,'' which opened Wednesday, she peers over the interviewer's shoulders and whispers, ``Don't move, there's a strange man sitting over there.''

The 10-year-old actress continues to spy on the stranger, maneuvering her body to avoid detection, and wonders aloud if the woman who just sat next to the man might be an enemy agent sent to receive sensitive government documents. Or perhaps she is a lover on a secret rendezvous in a swank Los Angeles hotel. Or just maybe the woman is a celebrated European actress meeting discreetly with a Hollywood producer to discuss her next film.

It doesn't really matter who they are; the fun part is spying on them. That's what Harriet would think and that's what Michelle thinks.

``I read those books (upon which the movie is based) when I was still in the fourth grade, which was long before I was asked to be in this movie, and one of the things I loved about Harriet was her spying,'' the fifth-grader said.

``I have always liked to spy on people, and I even saw somebody across the street in their underwear once. But that isn't the only thing I have in common with Harriet, though. I also dress like her, and I love tomato sandwiches.''

The casting of Michelle Trachtenberg in the part of Harriet would seem a natural. Who could argue with a shared love of tomato sandwiches?

But for Nickelodeon, the children's TV channel venturing into the movie business for the first time with ``Harriet the Spy,'' the casting had more to do with Michelle's acting talent and less to do with tomato sandwiches.

``She is simply a great actress,'' said Debby Beece, president of Nickelodeon Movies. ``We looked at 350 girls, but there was no question in our minds that Michelle was the right girl for the job.

``Harriet is a well-defined, fully developed character, and we needed a fully developed young girl to play her. We needed someone to cover the range of emotions Harriet has to go through and who could appeal to both boys and girls.

``Michelle comes off as genuine because she really is a genuine kid. Everyone can identify with her.''

Beece said Nickelodeon has been preparing to go into the movie business for a decade and selected author Louise Fitzhugh's ``Harriet the Spy'' for its initial project because it best represented what the channel is trying to accomplish.

``I loved the books as a kid,'' said Beece, 40, ``and I still love the books. There are so many parents around who enjoyed these books when they were kids and now can share that enjoyment with their own kids.

``The books reflect Nickelodeon's vision of kids as complete individuals and not one-dimensional, stereotypical things. The books also reflect Nickelodeon's philosophy of celebrating childhood.

``We are totally committed to the movie business,'' she added. ``It is a natural extension of what we do at Nickelodeon and helps to answer a problem we see in the marketplace. We believe that there are not enough quality family films being made.''

Michelle, who attends a class for gifted children at her Brooklyn public school, is not new to show business, although this is her first feature film.

She was appearing in newspaper advertisements before she could walk and performed in her first TV commercial at age 3. The product was Wisk, and she can still sing the jingle that played on the commercial.

``I saw a friend of mine in a commercial,'' she said, ``and walked over to my mom and said, `Mommy, I want to be on TV.' ''

She went on to appear in more than 100 commercials - Panasonic, Fuji, Kraft and Hoover, among them - and then made the leap to episodic television in ``Law & Order,'' playing a dying child on one episode. She has had a continuing role for two years on the daytime soap ``All My Children,'' playing an autistic youngster.

``That is an especially challenging role because I can't talk much,'' she said, making fun of her ability to talk about anything and everything.

The young actress stars in the movie as a curious 11-year-old who spies on people and writes her observations in a private journal. Unfortunately, the journal, which contains painfully honest thoughts about her closest friends, falls into the wrong hands, and Harriet is in danger of losing everything dear to her.

Rosie O'Donnell co-stars as Harriet's influential and ever-present nanny, and Vanessa Lee Chester and Gregory Smith play her best friends.

``I am naturally curious like Harriet, and I see a lot of myself in her,'' Michelle said. ``But I also learned a lot from the books. I learned about the value of friendship. I learned that a good friend is hard to find and you should never let them go. I also learned that you should always tell the truth, unless it might hurt someone.

``Like I really hate that shirt you're wearing,'' she said to the interviewer, ``but I don't want to hurt your feelings so I'll tell you how cute that shirt is. See what I've learned?''

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Photo: ``One of the things I loved about Harriet was her sp ying. I have always liked to spy on people. ... I also dress like her, and I love tomato sandwiches,'' says ``Harriet the Spy's'' Michelle Trachtenberg.