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 girlfriend Cameron Diaz went after one actress Scarlett Johansson. And she 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.”

What happen to Cameron Diaz and Justin? Or better yet Josh and Scarlett? Well these two look hot together, back in the Justin and Britney days.