Joel and Benji Madden are standing in front of a beat-up car, upset that their ride isn't worthy of going clubbing in, when who drives up but Hilary and Haylie Duff?

The two girls, oblivious to the fact that the two guys are not valets, hand them the keys to their flashy red convertible and let them drive off. In any other world, the Duffs would of course immediately recognize the Good Charlotte members — Joel is (duh) Hilary's boyfriend — but this case of mistaken identity is a little inside joke in the Duff sisters' new movie, "Material Girls."

"They get to steal our car!" Hilary chirped. "That was awesome!"

Hilary came up with the bit part for her boyfriend during a brainstorming meeting two months before "Material Girls" — the story of two cosmetics heiresses, the Marchetta sisters, who lose their company and have to deal with the consequences — started shooting last year. Director Martha Coolidge ("Valley Girl," "Rambling Rose") was figuring out the casting, which would include Anjelica Huston, Lukas Haas and Brent Spiner.

"There were these two roles for these gangster-looking dudes," Haylie explained, "so we said to Martha, 'What about Benji and Joel? They have tattoos.' "

"I'm quickly looking through the script, thinking, 'Ohmigod, what part is it they could play again?' " Coolidge recalled. "I wish it could have been a bigger part, but they were great. They knew what the whole movie was about."

The Maddens were already clued into the poor-little-rich-girls plot from the Duffs, but their sense of what "Material Girls" was trying to accomplish was aided by hanging out on set as well (see "Duff Sisters Living Large In 'Material' World"). But you won't see them in the party and club scenes — one of which features guest appearances by real reporters playing the journalists covering a press line at an event hosted by the Marchetta sisters. Olympic track star Carl Lewis also compliments Hilary's character on her "runner's legs" for another in-joke. "I don't know who's going to catch that," Coolidge said, "but that was funny."

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The Madden cameos, however brief, actually kick off a significant plot point. After the pair swipe the convertible, the newly impoverished Marchetta sisters are reduced to walking, renting cars and (heaven forbid) taking the bus, causing Hilary's character, Tanzie, to fume, "Friends don't let friends take public transportation!" It's an endless source of frustration for the two girls, who until that moment never had to live like mere mortals. But if someone had to do the dirty deed, Hilary is glad it was her beau and his bro.

"It was great, they were great," she said. "They got to goof off and make up their own lines, and they wanted to keep going, but we were like, 'We have other shots to do, we have to move on!' "

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