For Ali Larter, the Schwab’s soda fountain counter of the new millennium can be summed up in three little words: whipped cream bikini.

In 1999’s Varsity Blues, when Larter stepped out in a Reddi-wip surprise festooned with cherries for James Van Der Beek’s edible enjoyment, a star was born.

Since turns in Final Destination, Jay and Silent Bob Strike Back and Resident Evil: Extinction, Larter is now best known as the schizophrenic superhumanly strong Niki/Jessica Sanders/Tracy Strauss on “Heroes,” but she returns to the big screen in Obsessed, a movie she describes as “a kiss to the sexy thriller genre [like] The Hand That Rocks the Cradle, Single White Female and Fatal Attraction.”

In the film, she plays Lisa Sheridan, an office temp who develops a dangerous fixation on her superior (Idris Elba). As her passion spins out of control, she begins wreaking havoc on him and his wife, played by Beyoncé Knowles.

Larter says she delighted in the chance to tackle such a “delicious female villain” and that the role fulfilled a lifelong fantasy for her to step into the role of a “femme fatale.”

“The movies I love,” she explains “are Barbara Stanwyck in Double Indemnity, Rita Hayworth in Gilda; the real classic femme fatales are one of the reasons I always wanted to get into this business.”

When she discovered she’d be playing opposite Elba and Knowles, she signed on immediately.

“I have so much respect for Idris,” she gushes. “He was incredible on ‘The Wire.’ And Beyoncé is such an amazing talent.”

What she wasn’t prepared for were the demands of shooting a film and a TV show simultaneously. Although she says the producers of “Heroes” did their best to accommodate the film as much as possible, Larter still found herself in a maelstrom of work.

“I was shooting ‘Heroes’ at the same time [as Obsessed] so, for several months, I was working seven days a week, 12 hours a day,” she says.

“Sometimes I was driving from one set to the next. I was so tired, I think it fed into the delusion of the character,” she laughs.

For Larter, the greatest challenge of the role was making sure she wasn’t playing a crazy person in a crazy way. She needed the audience to feel like she was completely justified … if off her nut.

“To Lisa, it has to be completely rational,” she says matter-of-factly. “We wanted to make sure Idris’ character gives her enough reason to believe that he’s in love with her.”

Getting into the mindset of her deluded, lovesick, fanatical character forced Larter to draw on her own experiences and that meant dipping into the well of awkward high school memories for insight.

“When I was younger, I remember when I’d sit by the phone ALL NIGHT waiting for the guy to call,” she giggles. “If I could will that phone to ring, oh, I would have done anything! And there are games you play in your head. I think the obsession is the stories you tell yourself. Someone may say one thing, but it’s how you misinterpret it to hear what you want to hear.”

Moving out of high school and into the film’s corporate office setting, Larter says it makes perfect sense to set a thriller there since that’s where so many insecurities, doubts and suspicions can arise.

“People go to work and spend more time with their co-workers than they do with their family,” she points out. “This draws on those fears about what’s really happening when they leave the house and go to that office.”

Obsessed releases in theaters April 24.