"When we were sitting there, we were like, 'We know two guys with tattoos that would really be really funny.'" Haylie says.
"So then we asked Benji and Joel to do it and they made us laugh, take after take."
"They're really funny together, you know?" Hilary says, turning to her sister. "They're twins, so, I mean, Haylie, you and I act crazy and they act 10 times crazier."
"The two of them can crack themselves up for hours," Haylie agrees.
The Duff sisters, who share a Los Angeles apartment, are not unaccustomed to working together. Growing up in Houston, they began traveling in the Cechetti Ballet, when Hilary was just six years old. Quickly cast in TV movies, Hilary jumped on the fast track to success, and with the "Lizzie McGuire" TV show she became queen of the burgeoning "tween" market.
Planning for "Material Girls" actually started two years ago, but Hilary has been working at breakneck speed since leaving "Lizzie McGuire" two years ago. She's since appeared in two "Cheaper by the Dozen" movies, "A Cinderella Story," "Raise Your Voice" and "The Perfect Man."
She's also managed to parlay "McGuire Mania" into a singing career, first with "Santa Clause Lane," her single on "The Santa Clause" soundtrack, then with "Why Not" and "I Can't Wait" on "The Lizzie McGuire Movie" soundtrack, which went platinum.
2003's "Metamorphosis" topped the Billboard 200 album chart and yielded the hit single "So Yesterday." Her self-titled follow-up album, "Hilary Duff," reached No. 2. And by the time she reached her 18th birthday, she was touring to promote her "Most Wanted" greatest hits album.
After a whirlwind 2005, it's no wonder Hilary told MTV last November that she was simply worn out. "I think it's the type of exhaustion that, like, one night of sleep doesn't fix, you know? I was supposed to take September off for my birthday," she says. "But it didn't happen."
Now, she says, she's rested and ready for a burst of activity. "I lead a good life. I got to take half a month off," Hilary says. "I feel really good."
Hollywood can be especially hard on teen actresses as they look to make the jump into adult roles. Lohan, who is 20, tried to make the jump this summer with the romantic comedy "Just My Luck," a role that earned her $7.5 million, making her one of the top-paid actresses in Hollywood.
Lohan also appeared in Robert Altman's "A Prairie Home Companion," acting alongside Meryl Streep and Lily Tomlin. But both those films opened to mixed reviews and poor takes at the box office.
Hilary has been a little more cautious in choosing her parts. In June New York Times movie critic Stephen Holden slammed her for not taking challenging roles, and Duff immediately went on the defensive.
"He doesn't really fit the demographic, so I could really care less," Hilary told Elle magazine. "Look at me, and look at where he is -- sorry! Would he prefer that I take some superadult role that is inappropriate so I would have no place to grow?"
Now, as she prepares to deliver that same teen girl demographic in "Material Girls," she has a decidedly softer outlook, pointing to Natalie Portman and Rachel McAdams as actresses she admires, "not just for their abilities but for the work they choose."
But Portman is 25, and McAdams is 29, and Hilary says she's got time before she takes roles that require nudity and coarse language.
"I'm 18, and maybe I act a little older than my age, because of my job, but that doesn't mean I want to do some crazy role that maybe would be more appropriate when I'm 25, because when I'm 25, I want to have something to look forward to," Hilary says. "I've got time to play those roles when I'm pregnant, just not yet."
And then, Haylie interrupts her. "What's wrong with playing a character who's pregnant?" she says, giggling, and reminding her sister of her part on "7th Heaven."
"Oh," Hilary says. "Not that there's anything wrong with that."