Candice King on Bonding with Her ‘We Were Liars’ Sisters Mamie Gummer, Caitlin Fitzgerald (Exclusive)

It wasn’t hard for the Sinclair sisters to bond before filming started on We Were Liars.

In the upcoming Prime Video adaptation of E. Lockhart’s beloved book, Candice King, Caitlin Fitzgerald and Mamie Gummer play the three Sinclair sisters, Bess (King), Penny (FitzGerald) and Carrie (Gummer), whose children, the titular “Liars,” make some life-changing decisions one summer.

The actresses first met after they were cast, and King says they developed their chemistry off-screen quite quickly.

“It was very easy to bond with Mamie and Caitlin,” King tells PEOPLE on the red carpet at the June 10 Tribeca Film Festival premiere of We Were Liars in New York City.

King says she was able to bond quickly with her costars over motherhood, as they all have young kids

“We laughed the first day that we met at the production office [and] the next stop that we all made together to get to know each other was to go to Walmart because we had to buy summer toys for our kids.”

Looking back on it, she says there’s “really no [better] way to bond than going to a giant superstore with a bunch of new, amazing women.”

Afterward, they were “hanging out in the car and having some girl therapy for a few hours” together, which connected them even more.

Both King and Gummer are mothers of two. King shares daughters Florence and Josephine with ex-husband Joe King, while Gummer shares son Peter and daughter Mary with her ex-husband Mehar Sethi. Meanwhile, Fitzgerald welcomed her first child with husband Aidan Turner in 2022.

Fitzgerald says that their real-life connection helped them play around with how vicious their characters are to each other onscreen

Though the women play sisters who are most often at each other’s throats rather than empathizing with each other, Fitzgerald, 41, tells PEOPLE the trio are “much nicer to each other in real life than we are on television.”

“I think we were able to be so unkind to each other [on camera] because it felt like a really safe space,” she says on the red carpet.

“They’re such great girls,” the Masters of Sex star adds of King and Gummer, 41. “We had so much fun.”

When it came to playing Bess, who is keeping plenty of her own secrets over the course of the summer, King says there was a “lot of family drama to unpack”

“It’s a very sharp relationship,” she says of her character’s dynamic with her daughter Miren, played by Esther McGregor. Miren is one of the four titular “Liars,” along with Cadence (Emily Alyn Lind), Johnny (Joseph Zada) and Gat (Shubham Maheshwari). “I feel like that’s the only word that pops into [my head]. It’s a cutting relationship.”

“[Bess is] always trying to kind of curate her life, and if she can curate it enough on the outside, maybe she’ll find joy on the inside, which, according to therapists, is not the way to find joy,” King quips of her character.

The upcoming series takes place over the course of two summers and follows Cadence (Lind) and her “tight-knit inner circle, nicknamed the Liars, during their summer escapades on her grandfather’s New England private island.”

“The Sinclairs are American royalty—known for their good looks, old money, and enviable bond—but after a mysterious accident changes Cadence’s life forever, everyone, including her beloved Liars, seems to have something to hide,” reads the official synopsis.

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We Were Liars premieres Wednesday, June 18 on Prime Video.