The "Simple Life" star gets into a catfight when she returns to TV.

ET earlier reported that Nicole Richie will play a super villain in "Chuck" when the spy drama returns for its second season on NBC, but now we have the inside scoop from the set of what really happens!

It turns out that Nicole's character went to high school with super spy Sarah (Yvonne Strahovski) and Sarah is given the job of spying on Nicole at their high school reunion. Things go awry and the two gorgeous gals wind up having a showdown in the shower!

"[Nicole] was really on it," Yvonne tells ET. "She has a dance background, so her choreography skills were really great. It was one of the easy ones to film."

That said, things did get soggy: "At first it was a little cold and then it would warm up," Yvonne continues. "It probably took a few hours to shoot. The fight scenes usually take a while to shoot because we have to get the stunt doubles in to do a take."

Both Nicole and Yvonne play themselves in flashbacks in the episode -- back to when they were 17 and Nicole's character was a "mean" girl.

"Nicole was the preppy, good-looking, bitchy girl in high school, so they just put her in a pony tail. She was like the cheerleader," explains Yvonne, who underwent more of a makeup transformation. "I felt so sorry for myself because I looked really nerdy. I couldn't even look at the photo they put up of me at this reunion. And the braces brought me back to when I had braces."

Nicole will star in the fourth episode of "Chuck" when it returns to NBC this fall on Monday nights at 8 p.m.