When Justin Timberlake, 25, was casting the femme fatale for his new $1 million video, “What Goes Around,” the singer – who was on the rocks with 34-year-old girlfriend Cameron Diaz — went after one actress: Scarlett Johansson. And Johansson readily agreed, an odd choice for the seasoned thespian who’s known for her careful film choices.

But not if you have a crush on the leading man, says a pal of Johansson.

“Scarlett has always liked Justin. They’ve always had a flirtation – that’s why Cameron was threatened by her.”

With good reason. The decision to cast the 20-year-old as the video vixen pushed Timberlake’s relationship with Diaz to the breaking point, and paved the way for Johansson to move in for the kill.

“She could not believe he went behind her back to put her in the video,” says a Timberlake pal. “Cameron was very jealous of Scarlett.”

Adds the source, “they definitely got together after she did the video.”

Indeed, sources in both stars’ inner circles tell Us the couple are in the beginning stages of a relationship and excited to get to know each other.

Where did Cameron’s long-standing rivalry with Scarlett begin? How is Cam coping with the Justin’s newfound romance? What happened during Justin’s sexy encounter with Kate Hudson when he attended her New Year’s Eve bash? For more details on Cameron and Justin’s split and his budding romance with Scarlett Johansson, check out the new issue of Us Weekly, on newsstands this week?

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This is my favorite part of the cover…how funny is THAT!