Eric Charbonneau/Invision for Lionsgate/AP
November 29, 2015 02:10 PM

Their first day on set together was a literal nightmare, but after six years playing sisters in The Hunger Games, Willow Shields and Jennifer Lawrence began to feel like real-life family.

“My first scene with Jen was the nightmare scene where I wake up crying,” Shields, now 15, tells PEOPLE. “That’s a very emotional scene, and it kinda breaks the ice right away. I mean, you’re lying in bed together crying.”

Fortunately for the actress, who was just 10 years old when she signed on to play Primrose Everdeen in the blockbuster franchise, Lawrence was quick to ease the tension.

“Jen just made a ton of jokes, which made it a lot easier,” she says. “She also was like, ‘It’s okay if you can’t cry after like the 10th take – I can’t either!’ So she was really sweet.”

Lawrence became a valuable mentor to Shields on the first film, and their relationship evolved as she grew up on set.

“We started with me being really young and her being like my big sister,” the actress explains. “As I got older she became more like a best friend to me.”

Hanging around the older kids made Shields feel like she was growing up fast. Not only did she shoot up “5 or 6 inches” between the first two films, she learned maybe a little bit more than she should have listening to jokesters like Lawrence and Josh Hutcherson.

VIDEO: Do You Know the Difference Between a ‘Hunger Games’ Character and a Piece of IKEA Furniture?

“Jen and Josh know how to be professional in their scenes but have a lot of fun,” says Shields. “But honestly, the majority of the jokes I can’t even say! But it was hilarious, and it made it so much fun. I grew up quickly in that sense too!”

While Shields says her costars initially attempted to censor themselves, they eventually decided she’d just have to get used to it.

“I think the first few movies, they tried to lay off the super inappropriate jokes and we even had a swear jar for a while,” she explains. “But after that, it was just like, ‘Oh, you’re 13 and a half. Whatever. You’re old enough now! You’re basically an adult!’ ”

Finishing the final film, The Hunger Games: Mockingjay, Part 2, was an emotional experience for the famously tight-knit cast, and it was especially difficult for the onscreen siblings. On her last day of filming, Shields says Lawrence came to her trailer and said, “You know, we’re not gonna cry about this situation. It’s fine. We’ll see each other a ton.”

To commemorate their friendship, Shields says “Jen gave me a necklace. It has a little star on it. I wear it a lot.”

For more on the franchise, check out PEOPLE’s special edition THE HUNGER GAMES: INSIDE ALL FOUR MOVIES on sale now.

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