Sarah Michelle Gellar and Patrick Dempsey totally transform in exclusive Dexter: Original Sin photos

You've never seen Patrick Dempsey and Sarah Michelle Gellar like this.

The iconic TV stars have completely transformed for Dexter: Original Sin, and Entertainment Weekly has your exclusive first look at their '90s Miami cop makeover for the Dexter prequel series.

Set in 1991 Miami, 15 years before the original show, Dexter: Original Sin follows the titular Dexter (originally played by Michael C. Hall, but portrayed here by Shadow and Bone's Patrick Gibson) before he became the prolific serial killer. When Dexter's bloodthirsty urges can’t be ignored any longer, his father Harry (Christian Slater, taking over for James Remar) helps him adopt the Code designed to help him find and kill people who deserve it, keeping his younger sister (originally portrayed by Jennifer Carpenter, but now played Molly Brown) in the dark.

EW's first-look photos reveal Grey's Anatomy alum Dempsey is rocking a mustache and slicked back hair as Captain Aaron Spencer, head of the Miami Metro Police Department's homicide division, while Buffy the Vampire Slayer and I Know What You Did Last Summer alum Gellar dons shoulder pads as Tanya Martin, the CSI Chief. They're Dexter's new bosses as he begins a forensics internship at Miami Metro PD, and the actors both loved getting into the wildly different fashion and hair for their characters in the '90s-set series.

"It's funny — I think it's a different '90s than what I knew because I was like, 'I don't remember this hairstyle,'" Gellar says with a laugh. "But I played a teenager in the '90s, not an adult in a business environment, so maybe that is how people dressed in the '90s. Maybe that is how they wore their hair. They wore a lot heavier makeup in the '90s in Miami, oh boy. It's a lot of blue eyeliner and blue mascara. It's a lot of shoulder pads, and I like rocking a good pair of shoulder pads — it's the closest I'll ever get to looking like a linebacker."

Dempsey's new facial hair is totally real, and he was the one who suggested that Captain Aaron Spencer should have a mustache.

"I have to tell you, it was discovered accidentally," he admits to EW. "I was on holiday and I hadn't shaved in a while. I shaved my beard off because we were getting ready to come home and I kept the mustache going, thinking, 'Maybe I should do this,' because I was looking at a lot of vintage training videos of Miami police in the '70s and they all had mustaches. And then my hair was long and I slicked it back and I sent the picture to makeup and hair and they loved the idea, and that's how it started."

Dempsey knows that this look is "completely different than anything else" he's ever done before, and that was the point. "This is what I'm looking for, to do something where I'm able to physically transform," he says. "I've been fortunate enough to have a good long career and create a character that is beloved, so it's nice to have an opportunity to come in and [do something totally different]. Hopefully the fans will embrace this character — I think it helps, changing the look, so that people will look twice and go, 'Oh wait, what's going on here?'"

In fact, the physical transformation helped Dempsey get into the mindset of his character, both in the '90s storyline and in flashbacks to the '70s. "It's fun to change the look, and it's fun to play a cop and a homicide detective," he says. "I haven't done that."

Meanwhile Gellar — who freely admits she's a "basic b----" for thinking John Lithgow's Trinity Killer is the best season of the original Dexter — says her role is to be the "comic relief" on the prequel. "It's fun to sort of inadvertently be the character that teaches Dexter everything that he needs to know to really be a good serial killer, but all the while not knowing that you're doing it," she explains.

But Tanya isn't just someone to laugh at — she's also an extremely driven woman who has had to fight to get to where she is in her career. "In the early '90s it was very rare to have a female head of a department, let alone a female head of a department at Miami Metro," Gellar says. "So she has to be really tough and a little bit one of the guys to be able to have made it that far and have thick skin."

Gellar loved sinking her teeth into this role because "it's a very different kind of character" than she's used to portraying. "Normally, even though I play a lot of badasses, they always have this very sensitive side," she says. "And I guess she has one too. We're coming into an episode where you'll learn about her love for animals. But she's definitely had to work harder to get where she is than everybody else there."

Captain Aaron Spencer had a much different path to the top. According to Dempsey, he and Dexter's father, Harry, are best friends, "and there's a brotherhood there that goes back to the beginning of their careers. They've come up together. Aaron turns to Harry because he trusts him the most. And Aaron has a lot of respect within the group because of his wisdom and his humility."

Bringing Aaron and Harry's friendship to life onscreen couldn't have been easier for Dempsey and Slater, who, just like their characters, have known each other for their entires careers after starring together in the 1991 film Mobsters. "It's funny because Christian and I worked together in the early '90s, so we were laughing and reminiscing about still being survivors in this business after 30 years," Dempsey says. "We laugh at our former younger selves and how ridiculous we were and where we've come from. We're going back to a time when we were, I guess, about the same age as young Dexter, so it's been fun to flash back and you realize just how fast time flies."

Meanwhile Gellar is "living [her] teenage dream" working on a set with Dempsey and Slater. "I was talking to my high school best friend the other day, and I was like, 'Could you imagine if someone would've told us when we were in high school that one day I would be spending every day with those two?'" Gellar says with a laugh. "And she was like, 'No, we would never have believed it.' Can't Buy Me Love and Heathers and Pump Up the Volume, I mean, that was my teenage years!"

But it's time to talk about the bloody, plastic-wrapped elephant in the room. Aaron and Tanya are both at the top of their field as they investigate murders, so how do they not realize there's a fledgling serial killer emerging right under their noses?

"He's known Dexter since he's very little, he's seen him grow up," Dempsey says. "My job is to figure out what's going on as the detective, and oversee what the detectives are doing. [Meanwhile] there is the father-son relationship and dark passenger and how he's trying to navigate that. As the series progresses, that voice gets louder and louder and it's harder and harder for Dexter to control that."

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Since both Dempsey and Gellar are playing new characters in the franchise, unlike the rest of the characters who appear 15 years later in the original series, it's very likely they could end up at the point of Dexter's soon-to-be-favorite knife. "I've only signed on for one year, so who knows?" Dempsey teases. "I love being on the show, but you never know what could happen."

"It's Dexter — everybody should be worried," Gellar quips. "Even some characters in the first one that you never thought would be victims of his wound up being his victims. So can't ever be too careful on a show like this."

That's something that Gellar and Dempsey joke about all the time to each other. "I've learned my lesson about what it takes to be the character whose name is in the title," Gellar jokes. "It's really fun to not be the character whose name is in the title. How nice is it to not have all the pressure?"

Dempsey agrees, adding, "After coming out of being on shows for a very long time, the one-year commitment is something that you can deal with because you want to balance out your life, and this is a nice balance, being part of an ensemble. We're very happy that Patty's carrying it, and he's a fantastic Dexter."

Dexter: Original Sin is slated to premiere in December on Paramount+.