Hilary Duff, Almost 19

Actress and singer Hilary Duff appears backstage during MTV's "Total Request Live" show at the MTV Times Square Studios, Tuesday, Aug. 15, 2006, in New York.
AP Photo/Jeff Christensen
Hilary and Haylie Duff are sisters and best friends. That's why working on the film "Material Girls," which opened Friday, was more fun than it was work.

"Working on set together makes it much easier, because we have each other there for moral support," Hilary told's Karina Mahtani. "We just have a lot of fun together."

"We've grown up so close our whole life," added Haylie. "We're best friends and we have been forever."

They agreed that they could be direct with one another in a way they couldn't with other actors.

"We would be able to say to each other 'I need more of this' or 'you need to give me more of that,'" Haylie said.

"You can't say that to another actor," laughed Hilary.

Like most siblings who are close in age, Hilary, 18, and Haylie, 20, have a few little pet peeves about each other.

"We always, will be, like, waiting (for) each other," said Hilary. "I feel like I'm always waiting (for) her."

"Or, like, we fight about who's gonna drive," offered Haylie.

"You always want to drive. Why?" said Hilary, turning to her big sister.

"Because I'm a better driver," said Haylie, matter-of-factly.

"She thinks she's a better driver," said Hilary.