It's on! Beyonce and Ali Larter face off in their new thriller 'Obsessed,' in theaters Friday, and our own Kevin Frazier gets the lowdown on their onscreen throwdown!

"[My character] starts off in a great relationship with her husband -- very charming, sweet -- but when tested, she gets really gangsta," says Beyonce, who admits that in real life she's been known to break out her inner Sasha Fierce. "I'm a professional person, so I don't really get too gangsta, but I am a businesswoman -- and I am quite gangsta, I should say. So whenever I have to, in my own way, I have to turn on the gangsta, yes."

In 'Obsessed,' Beyonce plays Sharon, the glamorous and happy wife of Derek (Idris Elba), a successful asset manager who has just received a major promotion. Everything seems to be going their way until Derek's co-worker Lisa (Ali) becomes obsessed and begins to stalk him, putting everything they know in jeopardy.

"It was so much fun," says the singer-actress. "I had just finished playing Etta James in 'Cadillac Records' and it was so different. I gained weight for the movie, and a couple of weeks later I had to play Sharon, so it was hard for me to make the mental adjustment, the physical adjustment -- so I immediately went right into work trying to transform into this woman who basically fights for her family."

As for "Britain's Got Talent" overnight singing sensation Susan Boyle, Beyonce reveals that she's a fan!

"I really love her. I think she's very talented," she says. "I'm obsessed with a lot of great singers right now -- it's inspiring that we have so much good music and people that are into art and writing their own music. Absolutely, very inspiring."

Watch ET for more with Beyonce!