WITH AN EYE ON . . . : Tiffani-Amber Thiessen zips into ‘90210’ ready to defy expectations

There’s a new bad girl in Beverly Hills.

Tiffani-Amber Thiessen, long of NBC’s silly tween sitcom “Saved by the Bell,” replaces the errant Shannen Doherty as the newest vixen in Fox’s fabled zip code, which begins its new season Wednesday.

She won’t take over the role of Brenda Walsh, the sweet, if slightly neurotic, character the publicly sour Doherty played on “Beverly Hills, 90210.” She’ll play Valerie Malone, a family friend of the Walshes who moves into Brenda’s room and the lives of her friends.

For Thiessen, the transition has been easy.

“I knew the cast already,” she says, since her real-life boyfriend Brian Austin Green was already on the show. But what about working on the same set? “Our characters don’t get involved,” she says. But Valerie does get quite cozy with Dylan (Luke Perry), Brenda’s ex. As Thiessen tells it, Valerie moves smoothly into the “90210” storyline.

“Valerie’s described as a chameleon,” she says at a small Santa Monica coffee shop. “She’s totally different from scene to scene and from one person to the next.” When it’s suggested that kind of behavior might indicate Valerie’s not too stable, she says, “They haven’t said if she’s mentally imbalanced yet. But that’s how she feels safe. It’s the only way she knows.”

The “90210” producers don’t want to “reduce Valerie to ‘the bad girl,’ ” executive producer Charles Rosin says in a phone interview. “I don’t want her to be that stereotype. I hope everyone’s patient. We’re reinventing the show and finding her character. Our show’s always evolving and so’ll Valerie.”

For the moment, Rosin says, Thiessen “plays her perky and she plays her very dark. I don’t know her that well, but it doesn’t seem part and parcel of what she is, that dark side.”

“Valerie just does whatever she can to feel comfortable,” Thiessen explains, as she sits back in the oak chair and throws her army-booted foot over the arm of the chair, her long floral dress flowing around her. “She smokes pot a lot.” In her character’s defense, typical of actresses who play connivers (a la Heather Locklear’s “Melrose Place” nightmare, Amanda), Thiessen describes Valerie as “a very troubled lady.”

And much of the hype surrounding the new character dwells on how nasty she is. “I’d call her a bad girl in the sense that she does bad things,” says Thiessen. “She’s had a lot of difficulty in her life and doesn’t know how to deal with them.”

Thiessen, who’s had little difficulty in her own life, proudly says she has nothing in common with her character. A model since the age of 9, she was Miss Junior America and a winner of the Teen Magazine Model Search. Her parents, she says, “entered me in pageants to teach me poise and how to be a girl. It wasn’t to win prizes or do the beauty thing.”

The “beauty thing,” however, did lead to “Saved by the Bell” and its prime-time incarnation, “Saved by the Bell: The College Years,” which lasted four months. She stars in the upcoming movie spinoff “Saved by the Bell: Wedding in Las Vegas,” which airs Oct. 7 on NBC.

“I haven’t gotten everything I’ve gone up for,” she notes, lest it seem as if her life is too charmed. “I auditioned for ‘Models Inc.’ and didn’t get it . . . but something better came along.”

But does that something better mean she has big shoes to fill?

“I knew you were going to ask me that,” she says with a sigh. “No. I am not Brenda, I am Valerie. I don’t look anything like her,” the blue-eyed brunette says of Doherty, who also happens to be a blue-eyed brunette. “They may say Valerie is a lot like Shannen Doherty. Maybe she is. But she’s completely different than Brenda.”

“Beverly Hills, 90210” airs Wednesday s at 8 p.m. on Fox.