'The Summer I Turned Pretty' Stars Reveal How [Spoiler] Is 'Pivotal' to Season 2 (Exclusive)

Gavin Casalegno and Christopher Briney exclusively tell PEOPLE how a heartbreaking storyline plays out in season 2

Warning: This post contains spoilers for season 2 of The Summer I Turned Pretty.

Cousins Beach will never be the same again.

More than a year after its season 1 premiere, Prime Video’s hit series The Summer I Turned Pretty returned with a three-episode season 2 premiere on Friday, including the shocking revelation that Rachel Blanchard's Susannah has died.

Speaking with PEOPLE exclusively, Gavin Casalegno and Christopher Briney tease how that storyline plays out the rest of the season — and the impact her loss will have on the trajectory of their characters Jeremiah and Conrad moving forward.

Related: 'The Summer I Turned Pretty': A Look Back on How Season 1 Ended Before Season 2 Premieres

In the series adapted from Jenny Han's best-selling trilogy of the same name, the second season picks up almost a year after the season 1 finale, when it was revealed that Susannah had gotten sick again several years after being in remission from cancer.

As Belly (Lola Tung) daydreamed about basking in the sun at the summer house with Conrad and Jeremiah in the season 2 premiere, she quickly snapped back to reality in her school classroom and gave an update on her current life.

“I wasn’t in Cousins,” she said in voiceover. “Conrad and I weren’t together. And Susannah was dead. Nothing would ever be the same again.”

Much like Han 2010's novel, It's Not Summer Without You, the second season jumps back and forth in time, giving viewers a glimpse of how Susannah’s heartbreaking death affected each character.

While Laurel (Jackie Chung) was penning an entire book about her friendship with Susannah, appropriately titled It's Not Summer Without You, and Steven (Sean Kaufman) honored her memory with his graduation speech, Susannah’s death impacted her sons Conrad and Jeremiah in very different ways.

Related: 'The Summer I Turned Pretty' Season 2: Everything to Know

For Conrad, her death has played a role not only in his personal relationships — including his romantic one with Belly — but also in his life decisions.

“It's pivotal in his journey at this time in his life,” Briney, 25, tells PEOPLE. “It's almost impacting every single decision he makes right up until the end. It's a big thing in one's life so he's going through it.”

Though Conrad has always been inclined to keep his emotions bottled up at times, Briney notes that he does "admire his strength."

"It feels like sometimes he's unnecessarily strong when he could just be asking for help, but I admire that," he adds. "I admire his fortitude and his outward resilience."

Related: Where Was 'The Summer I Turned Pretty' Filmed? All About the Real Locations Behind Cousins Beach

On the other hand, the ever-optimistic Jeremiah was grappling with losing the one constant bright light in his life.

“I read in some research that I did that losing a parent is the second-hardest thing to work through and go through as far as heart pain, and the first is losing your child," Casalegno, 23, tells PEOPLE. "It really just shows you how important it is to have a mother in your life, and especially one as amazing as Susannah."

“She is all sunshine and rainbows and really the glue that keeps the Fisher family together,” he adds, noting that that loss also plays a role in his relationship with Conrad. “Without that, it can be pretty hard to keep the family dynamic.”

Though Casalegno notes that Han wanted to stay “as true to the books” as possible with season 2, including the death of Susannah's character, he teases that there are still quite a few surprises in store for book fans as the season continues.

“There's a lot of things in the show that are not necessarily in the book,” he says. “Even the most loyal of book readers are going to be surprised.”

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The first 3 episodes of season 2 of The Summer I Turned Pretty can be streamed now on Prime Video, and new episodes will drop Fridays through the Aug. 18 finale.

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