Don't sweat the pants. Granted, no pair of jeans -- no matter how stretchy -- could ever fit four friends of radically different sizes.

But as fans of "The Sisterhood of the Traveling Pants (search)" will tell you, the pants are the least of it.

"It's not about magical pants -- it's about this magical friendship," says 17-year-old Blake Lively (search).

"Most teen films these days make young women seems like such superficial, horribly back-stabbing people ... But there are girls out there with real problems. It's not just about getting the hot guy or being popular!"

Blake isn't only a fan -- she's in the movie.

Opening Wednesday, "The Sisterhood of the Traveling Pants" -- starring Blake, Amber Tamblyn (search) of "Joan of Arcadia (search)," "Gilmore Girl (search)" Alexis Bledel (search) and America Ferrera (search) of "Real Women Have Curves (search)," is the first of Ann Brashares' best-selling series to hit the big screen.

(The latest book, "Girls in Pants: The Third Summer of the Sisterhood," (search) came out in January and immediately hit No. 1 on the charts.)

Teens who've read the books and got a sneak peek at the film have given it a big thumbs up -- as have several men, who sobbed during the sad parts.

"If I can make a few grown men cry, I've done my job!" director Ken Kwapis jokingly told The Post.

The challenge, he said, was getting four young actresses who'd never met before to convincingly play lifelong friends.

And so, as a way of turning Blake, Amber, Alexis and America, respectively, into Bridget, Tibby, Lena and Carmen, Kwapis locked them in a room for an hour and a half.

"I told them that this was the last time they'd be alone together before the camera crew and the hair and makeup people came," he said.

He returned, 90 minutes later, and unlocked the door.

"But they chased me away," he said. "They just wanted to be together."

Once they'd broken the ice, Blake tells The Post, the girls were inseparable. "We ate together, we shopped, and we watched movies -- Harry Potter, 'The Stepford Wives' and 'Dodgeball.' We loved 'Dodgeball'!"

In fact, Kwapis says, the girls ad-libbed endlessly about the Ben Stiller/Vince Vaughn flick, and some of their jokes slipped into "Traveling Pants" ("Maybe someone can point out the references to me," he said, plaintively.)

Blake, who was 16 during the shooting, was the youngest of the four, the only one who'd never acted professionally before.

It helped, said the blond Californian, that she was so busy with homework and cheerleading practice that she'd never seen her co-stars' TV shows.

"So that cut down on the intimidation factor," she giggles. "But they made me feel so comfortable from the beginning."

It also helped, she says, that her dad, actor/director Ernie Lively, played her on-screen father. (Blake's siblings are actors, too; her mom is a talent coach.)

Wasn't it awkward, with her dad around, to play that scene where Bridget seduces her counselor at soccer camp?

She laughs. "It was awkward. But that was the character. It wasn't Bridget being trampy -- her mother had committed suicide and she had an emptiness inside her.

"She's looking to fulfill her life, but [sex] leaves her emptier. She made a mistake, and her friends helped her deal."

Move over, "Mean Girls" -- the Nice Girls are coming to town.