This weekend's big winner at the box office was "Obsessed," which took in an expectation-defying $28.5 million. The thriller pits Beyoncé Knowles' radiant new mother against Ali Larter's delusional office temp in a bruising girl-on-girl battle in which Beyoncé stands by her man by beating the crap out of the woman who tries to steal him away.

The two beauties eagerly dove into the chance for some fist-bashing, hair-pulling action during the film's climatic final fight. "We went for it like, 'You clock, I clock,' " Larter told MTV News. "A little head-butting, some kicks in there — we went for it. We got bruised up. We spent a week shooting it."

"I had so much fun, because I'm a lover — I'm not a fighter," Beyoncé told us. "But I got to fight!"

Larter came away impressed with the performance of the actress whose neck she tried to wring. "[Beyoncé's] amazing," she said. "She did it in four-inch heels."

The challenge for Larter herself — whose character, Lisa, develops an unhinged, ultimately violent attachment to her co-worker Derek, who happens to be married to Beyoncé's Sharon — was to portray a woman on the edge without going too far. "You never want to play crazy," she explained. "You have to play from a place of love.

"You have to believe that what she's doing in her head is completely rational," Larter continued. "Even though it may seem crazy to the outside world, to her she's falling in love with this man and he's giving her signals. She really sees them and believes that they're meant to be together. Her behavior asks the question, 'How far will you go for love?' "

For inspiration, she looked to classic femme fatales of films past, including Rita Hayworth in "Gilda," Barbara Stanwyck in pictures like "Double Indemnity" and Faye Dunaway in "Chinatown." "This is an homage to them," Larter said.

Did Larter draw on any of her own experiences to play the fatally attracted Lisa? "I could go crazy if someone went after my man," she admitted with a laugh. "I'm not someone who feeds into the jealousy side of it."

In the real world, though, Larter sticks by one important rule: "I don't compete with other women. I only compete with myself."

Check out everything we've got on "Obsessed."

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