Brandy Norwood, who portrayed Karla in I Still Know What You Did Last Summer (1998), had hoped to return as the character for the follow-up.
(23 September 2024) Freddie Prinze Jr. announced that he had agreed to return as Ray Bronson in the new sequel.
(13 December 2024) Jennifer Love Hewitt officially confirmed she will reprise her role as Julie James in the upcoming sequel.
In 2014, director Mike Flanagan and writer Jeff Howard announced plans for a reboot of I Know What You Did Last Summer (1997). In a 2016 episode of the "Shockwaves" podcast, Flanagan said the film would not have any connections to the 1973 novel or the 1997 film, and that the project would require an estimated budget of $15–20 million. The proposed film ultimately did not materialise, and instead a television series, I Know What You Did Last Summer (2021), was released.
Jennifer Kaytin Robinson has said the film will take into account the events of I Still Know What You Did Last Summer (1998).
Filming was carried out mainly in Sydney, Australia, with several sequences shot in California in early 2025.
Camila Mendes had initially been cast but later withdrew after her commitments clashed with Masters of the Universe (2026).
On 25 October 2024, Gabbriette was confirmed as a member of the cast.
Writer-director Jennifer Kaytin Robinson couldn't revive Sarah Michelle Gellar's Helen Shivers for this sequel. In a conversation with Entertainment Weekly, Robinson said she "tried relentlessly" to find a way to include Gellar in the new I Know What You Did Last Summer. Her efforts came to nothing because Helen Shivers is definitively dead in the story. "I tried, okay? I harassed her! But she is dead," Robinson admitted. "I even pitched some wildly out-there ideas — like, what if she wasn't really dead but living in hiding?" Robinson recalled Gellar responding, "I was on ice. I was the most dead a person could be. You can see my frozen body." Robinson added, "I was like, 'Yeah, but what if?' and she answered, 'I am dead. I am Sarah Dead Gellar.'"
Functions as a direct legacy sequel to I Still Know What You Did Last Summer (1998), and does not acknowledge the events of I'll Always Know What You Did Last Summer.
Director Jennifer Kaytin Robinson clarified how Ray and Julie remain alive despite the finales of the 1998 sequels, which appear to show their characters' deaths. "My approach to the franchise has been to regard those closing moments from the first two films as outside the canon. In the first film she is attacked in the shower through the glass door, and in the second she is pulled under the bed. So both characters are alive and well, and I would say we've continued that tradition in our film."
Director Jennifer Kaytin Robinson reflected on Freddie Prinze Jr, "I find Freddie so warm, affable and pleasant, yet there's an edge to him that I always thought was fascinating on screen when he was younger. There's an earthy kind of manliness that feels properly f*cking raw and brilliant. After we sat down for breakfast the very first time and spoke about all of this, I thought, "Oh, that's Ray."" Both Robinson and Prinze Jr. aimed to be faithful to who Ray might be now, and their personal connection (Prinze describes Robinson as a close family friend after his wife, Sarah Michelle Gellar, collaborated with Robinson on Do Revenge) had already established trust between them. The actor says that the pre-existing friendship meant he "could go wherever he needed me to go and know that it was the right thing." "I think what Robinson and I both brought to Ray was a far greater vulnerability in this film," Prinze added. "When the s*it hits the fan again, so to speak, you really see how these characters absorbed the trauma from the first two films, and now they're like a sponge being squeezed and you get to see what emerges. The amount of vulnerability we were able to put in felt honest and genuinely organic. Nothing came across as forced or phoney, or like a note someone had instructed us to hit."
Freddie Prinze Jr. spoke about his and Jennifer Love Hewitt's return, saying "we didn't need to lay out any backstory — it's all in the script, and viewers will immediately see where the characters are now and what they've endured." Director Jennifer Kaytin Robinson added, "without giving too much away, both of them have processed the trauma and become fully realised adults, but they cope with what happened in very different ways. That difference is the primary source of conflict between them: how do you move forward? Now that people have many more tools, do you use them all, or do you suppress things and just carry on?" Ultimately, Robinson explains the new film is about Ray and Julie finding catharsis by helping the younger cast. She believes they each harbour emotional wounds they've merely patched over, and those coverings are pulled away by the younger actors — this film is very much about them. Each of Ray and Julie is connected to a member of the younger ensemble, and the story revolves around helping all five of those youngsters work through their trauma. In doing so, Robinson thinks the pair are able to achieve closure and release, and move forward in ways they wouldn't have without experiencing this alongside the younger group.
Freddie Prinze, Jr. told Entertainment Weekly about his role and Jennifer Love Hewitt: "They were all traumatised. Reconnecting with Ray, you can see how that has moulded him into the position he's in now. He loves his town and, in his view, not everyone else does, and he's a touch grumpier because of how the world has treated him since. He's been through a great deal, and I don't know if he's handled it the way the modern man would, if you know what I mean. He's a bloke from the '90s like me, so I reckon he's probably bottled up far more of those feelings we've discussed." Prinze went on to add, "Ray runs a pub. It's essentially where he is in his life now. He's not the hands-on fisherman he thought he would be. He owns and manages his own place and is trying to keep it afloat."
Director Jennifer Kaytin Robinson said the younger cast were thrilled by the opportunity to work with the original stars: "The energy when both of them arrived on set was electric — it was such fun, and there was a huge amount of reverence and respect for them. It was an unbelievably cool, almost surreal experience. Seeing these younger actors, who genuinely care about the films and about working with Freddie and Jen, was truly brilliant to watch."
Director Jennifer Kaytin Robinson tackled the film from a contemporary perspective, aiming to examine how trauma moulds people. "I set out initially keen to probe: if something like that happened to you, how would it change you, and what sort of person would you become afterwards? So I really wanted to examine both Ray and Julie and ask, 'Okay, how did that event shape each of them and where would they be now?'"
