Spoilers below for “I Know What You Did Last Summer.”

Talk about a last-minute twist.

Sarah Michelle Gellar wasn’t initially meant to return as slain beauty queen Helen Shivers in the new “I Know What You Did Last Summer” sequel, according to costume designer Mari-An Ceo.

Ceo, who has also worked on other horror films like last year’s “MaXXXine,” revealed that Gellar was only on set to support her husband, Freddie Prinze Jr., who reprised his original role as Ray Bronson.

It was then that director Jennifer Kaytin Robinson, who is Gellar’s close friend in real life, and writer Sam Lansky approached the actress about reprising her role as Helen for a surprise dream sequence cameo.

“Sarah Michelle Gellar wasn’t supposed to be in our movie at all,” Ceo claimed exclusively told The Post. “She was there with her kids, with Freddie, to come and just hang out.”

“And then [Robinson], they’re very good friends, she said she wanted to do this dream sequence,” Ceo explained.

Gellar and Prinze, who married in 2002 and have since welcomed two children, co-starred together in the original 1997 “IKWYDLS.”

Despite being brutally murdered by the Fisherman shortly after becoming Croaker Queen of the 1996 Southport Beauty Pageant, Gellar makes a short surprise cameo in the sequel when Madelyn Cline’s character, Danica Richards, falls asleep at a police station.

Gellar’s Helen appears in Danica’s dream and taunts Danica while dressed in her full Croaker Queen costume.

But bringing Gellar’s dream sequence cameo to life was not easy, Ceo admitted. The new “IKWYDLS” was filmed in Australia and the Croaker Queen crown was either lost or destroyed after the original movie was finished filming.

“[Robinson] said she wanted to do this dream sequence, so we replicated her look for the dream sequence, but we needed the old Croaker Queen crown that didn’t exist anymore,” the costume designer explained.

“And we had to remake that in Australia, in like the smallest window of time,” she added.

Luckily, Ceo and her team were able to remake the crown in time to shoot Gellar’s cameo. Everyone on set then took turns trying the Croaker Queen crown on.

“It was beyond anything any of us could imagine,” Ceo recalled. “We got the box with the crown in it, and everybody had to put it on.”

“Sarah was like, ‘Oh my god, this is amazing. It’s even better than the original,’” she continued. “We were all excited. It became like Croaker Queen Day, and everybody had to come see the crown.”

“It’s just an homage to the original one, and I’m so glad we were able to do it.”

Gellar recently opened up about the surprise cameo when she shared several behind-the-scenes photos from her time on set.

“Spoiler alert! Helen Shivers is still dead,” Gellar, 48, began via Instagram on Wednesday. “When anyone asked me if I was going to be in the new movie, I never lied. I always answered ‘I am dead.”

“And honestly I didn’t see how I could be in the movie until [Robinson] and [Lansky] pitched me that crazy dream sequence with [Cline],” she added.

“I still can’t believe we were able to keep it a secret. Long live dead Helen Shivers the og Croaker Queen.”

Meanwhile, Gellar’s husband was just happy they were able to surprise audiences with the secret scene.

“We were all really excited for audiences to get to see that and get a little bit of extra love that they weren’t expecting,” Prinze, 49, told People after the movie’s July 18 release.

“It’s nice to know a secret, and to know how well-received it’s going to be, months and months before it ever goes out there,” he went on. “To give people another taste of that, I just think it was awesome. I’m glad everyone made it happen.”

Robinson recently broke her silence about Gellar’s surprise appearance.

“As a super fan of this franchise, I was never not going to put Helen Shivers in the movie,” the director told People. “And I didn’t lie – she is dead.”

“We wanted to bring Madelyn and Sarah together on screen, and I’m so excited about the results,” Robinson added. “I feel like it’s just so fun.”

Besides Gellar and Prinze, the new “IKWYDLS” sequel sees the return of Jennifer Love Hewitt’s Julie James.

Brandy Norwood, who portrayed Karla Wilson in the 1998 sequel “I Still Know What You Did Last Summer,” makes a shocking appearance in the post-credits scene, too.

As for the new cast members, Cline was joined by Chase Sui Wonders (Ava Brucks), Jonah Hauer-King (Milo Griffin), Tyriq Withers (Teddy Spencer), Sarah Pidgeon (Stevie Ward) and Gabbriette Bechtel (Tyler Trevino).

“I Know What You Did Last Summer” is now in theaters.