Jeremy Piven Kept “Staring at” Jessica Simpson’s Butt on Entourage Set

 Andrew Shawaf/Gaz Shirley/ (2)

Jeremy Piven found a way to pass time on the Beverly Hills set of Entourage Tuesday as Jessica Simpson filmed her buzzed-about cameo.

"In between takes, he was staring at Jessica's ass in the most obvious of ways," a witness tells "He wasn't being sly about it and would just watch her every move. He flirts with ugly girls, so you can imagine how crazy for Jessica Simpson he was. She almost seemed like she couldn't wait to change into her jeans, but she was a good sport about it and kind of laughed it off."

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Lucky for Simpson (whose acting has been panned in the past): Her scene "didn't require any acting," says the source. (The singer, 29, plays herself.)

"It was simple – she answers the front door, and it's Jeremy Piven," the source tells Us. "She had on a very tight skirt and was leaning against the door in a sexy way. They did the scene a few times and didn't change much. The whole production is very laid back and they always have a good time. Jessica seemed to be having a ball and was not taking herself too seriously."

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Her father Joe also was on set, a source tells Us, chatting up the producers. "They were buttering each other up," says the witness. "At the end of the scene, they all took pictures together for the set photographer and then Jessica went back to her trailer with her assistants. She brought a bunch of clothes in her Louis Vuitton luggage, loaded everything back up into her assistant's car and left."

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The shoot went well.

After filming, Simpson Twittered: "I have the utmost respect for the entire cast and crew of Entourage! Thank you for making the past 2 days memorable :)"

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