I Know What You Did Last Summer's big twist, explained by the showrunners

There are plenty of horror reboots and revivals coming out now, like Halloween Kills, Paranormal Activity: Next of Kin, and Scream. But, just because you've seen the original teen slashers before doesn't mean you know the rules of the remakes. There's still plenty of fun to be had within the subgenre — and surprises — as can be seen in the I Know What You Did Last Summer television series.

The Amazon Prime show borrows from the 1997 film classic of the same name — itself an adaptation of Lois Duncan's 1973 novel — as it follows five teens who hit someone with their car on the night of their graduation party. Horrified by the accident, they decide to dump the body in the water, only to be haunted by their actions one year later, when a killer starts coming after them.

But unlike the movie that came before it, the show is bringing things much, much closer to home as it translates the story for a much more modern audience. For one, the dead person is no longer a stranger to the group of teenage friends. Instead, it's Lennon, the more fun-loving, extroverted identical twin sister of the driver of the car, Allison, both of whom are played by Madison Iseman (Jumanji: Next Level).

However, that's not even the biggest twist of the show as none of Lennon's friends, who were in the car with Allison at the time, realise that it's actually her who is dead, simply assuming that it was Allison because Lennon had been wearing her jacket at the time, with both twins having previously attended the same party earlier that night. This leads to Allison making the split-second decision to not disclose who she really is to Lennon's friends that night and to just start living as her sister, essentially stepping into Lennon's identity for what could be the rest of her life.

"I always knew it was going to be one of the twins," showrunner Sarah Goodman (AMC's Preacher) tells SYFY WIRE. "I felt like I wanted Allison to live because she to me was the most mysterious. You think you know who she is, that she is the 'good' one, that she is more trustworthy because she wasn't part of that party scene. So it was much more interesting to me to see what she was going to do with being alive and the chance to become someone else."

The twist works for a few different reasons. Not only are all the friends still grappling with the fallout of what happened that night, but as they all go off to college and then return for the summer, it also means no one really has time to notice Lennon's "changed" and not really acting like herself. But the biggest reason the lie proves so effective is that Allison immediately comes home and confesses to her father who immediately starts helping her clear her tracks, as they craft the lie that "Allison" ran away, unable to face that she wouldn't be going to college as Lennon would be in the fall, or that she didn't have friends either.

"A parent is going to know which twin it is, so there's no way he's not going to know," explains Goodman of this next twist, which sets up the show's biggest secret (other than who the real killer is). "I know his response is surprising to people and I really had to ask myself, what would I do if my child came to me and was going to spend their life in jail and I'd just lost my other child? I don't know that I would make a different decision. I mean, I'm not saying I would make that decision. I just don't know."

She continues, "I felt for him in that moment and it didn't feel calculated. It didn't feel like 'Let's make up a lie.' It felt like an emotional decision that then they both had to live with and obviously changes their relationship as well."

One of the things that comes back to haunt Allison — other than a killer intent on killing her and the rest of Lennon's friends — is how little she really knew her twin, as she and Lennon had a rather contentious relationship when they were alive. One of the reasons for this is their mother's death, which is connected to a cult on the island. While Allison is still mourning their deceased parent, Lennon seems to have moved on.

"They can be so rough with each other and somehow, especially as twins, know that they're really all they have," says Goodman. "They're each other's worst competition. They remind each other of all the parts they hate about themselves. And they're also defined by each other. If one of them is one thing, the other has to be the other. So if Allison's in pain all the time, Lennon can't be in pain. If Lennon's an overachiever, then Allison has to be an underachiever. Those two sides of that same face is interesting. [But] people should know they do love each other, even if it doesn't feel like that all the time."

Still, who Lennon really was remains one of the bigger mysteries of the series, as Allison must uncover all her secrets if she's to successfully keep passing as her twin as the show goes on, and maybe even figure out who the killer is. But it's clear through the flashbacks interspersed throughout each episode, that not even Lennon's childhood best friends Margot (Brianne Tju, Light as a Feather), Johnny (Sebastian Amoruso), Dylan (Ezekiel Goodman), and Riley (Ashley Moore) knew her as well as they thought they did.

According to Goodman, that's by design.

"Lennon appears to be whatever you want her to be," she says. "To the adults, she's the valedictorian. She always has a smile on her face, and she makes her dad feel good about himself as a parent. She makes everybody feel special... At the same time, she's the biggest partier and that just because you can't see it on the outside, doesn't mean that she's not in just as much pain as her sister is or anybody else. That's a character that we don't always see. We see a valedictorian or a party girl or someone who's in pain. [But] Lennon is all of those things."

In this way, the series is very much about Allison trying to figure out who she is, while also coping with her guilt and keeping the spirit of her sister alive for everyone else around her.

"Ultimately the show and the book and movie are about how these kids who do this bad thing and how they live with it," says Goodman. "So in terms of the show, it's very much about who they were going into that night and who they came out of that night realizing they were. For Allison, more overtly than the other characters, this is her chance to be whoever she wants to be. I'm not sure if she knows that yet. That push-and-pull between being Allison and being Lennon or being something in between is very much a driving force for her character."

For anyone looking for any cameos from the stars of the film, Goodman says viewers shouldn't expect Sarah Michelle Gellar (Buffy the Vampire Slayer), Freddie Prinze Jr. (Star Wars Rebels), Jennifer Love Hewitt (Ghost Whisperer), or Ryan Philippe (MacGruber) to make any appearances on the show anytime soon.

"There are little Easter eggs and shout outs to the movie that I've scattered through for die-hard fans. But [the show] is a whole new world and a whole new time. It felt like it was a disservice to our characters' introductions and to the world's introductions to be looking for the original the whole time."

The first five episodes of I Know What You Did Last Summer are currently available to stream on Amazon Prime.