The Sisterhood of the Traveling Pants Director Reveals the Trick He Used to Help the Stars Become Fast Friends (Exclusive)

The Sisterhood of the Traveling Pants is 20!

The beloved film that followed four lifelong friends who remain connected by a shared pair of jeans as they go their separate ways for the first time one summer is celebrating its 20th anniversary.

The movie, based on the novels by Ann Brashares, also turned on-screen best friends America Ferrera (Carmen), Alexis Bledel (Lena), Blake Lively (Bridget) and Amber Tamblyn (Tibby) into real-life friends, too. Speaking with PEOPLE about the film’s anniversary, director Ken Kwapis recalls seeing their bond blossom and helping it along.

Sharing a story that also appears in his book, But What I Really Want to Do Is Direct: Lessons From a Life Behind the Camera, Kwapis recalls knowing early on that the actresses were perfect fits for their respective characters.

“I met a lot of good actors, but I feel like the moment I met each of the four, that was it,” Kwapis tells PEOPLE.

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“It’s hard to imagine anyone else other than Blake Lively as Bridget, even though it was her first acting job. I mean, she was 16 when she came in to audition and yet she won it in the room. She had such a sort of intuitive connection to this character.”

“And I felt the same [about the rest],” he continues. “Amber and Alexis and America obviously had more experience. Amber and Alexis were in hit TV series. America had just come off of a fantastic independent feature, Real Women Have Curves, so they had more experience. But [it was] the same thing when I met with each of them. It was clear that they had ‘it’ — that was it. We were done looking.”

The actresses mastered their characters but with little time to bond before filming began, Kwapis wanted to make sure there was also a friendship formulating that would help bring the characters’ bond to life.

“The challenge was that the four actors had never met before this film, and the characters don’t have a lot of scenes together. Basically the four of them are in scenes at the beginning and the end. They bookend the film,” Kwapis notes.

“So the challenge for me in pre-production was how to take these four actors toward each other and turn them into lifelong friends quickly. And so one of the things I tried to do is create certain exercises that would help the four of them bond. I mentioned this one in my book, that I sent the four actors off to a thrift store with, I don’t know, $75 or $100 apiece with the instructions to buy something in character and to also advise the others in character as well.”

“It was kind of an interesting way to rehearse without actually rehearsing a scene,” Kwapis continues. “It was an exercise that gave them a chance to create a shorthand that they were able to use to, again, create that sense that these were friends from birth.”

Not only did they do that, but they formulated a friendship that the four actresses continue to cherish to this day.

“It’s so amazing and it’s particularly rare in show business. I think that in show business people have intense relationships for a short time and then go their separate ways. But in this case, these four actors came together, played four best friends and then have remained best friends. So actually, it’s rare and wonderful,” Kwapis says.

He, too, stays in touch with the cast and crew that made the film possible.

“I’m definitely in touch with everyone. I mean, not on a regular basis, but for instance, last year there was a screening of the film in New York at the Roxy Cinema and I spoke after the screening along with Amber,” he shares.

“I’m definitely in touch with the others now and then and, again, I feel like it’s been wonderful to become friends with all of them and I hope to work with them again.”

The Sisterhood of the Traveling Pants is streaming on AppleTV and Amazon Prime.