When it came to onscreen chemistry between these A-list stars, it turns out they weren’t really acting after all.

That’s right.

Whether its Ione Skye and John Cusack’s summer romance in Say Anything that, years later, would lead to a post-divorce hookup or Lea Michele and Matthew Morrison who dated for a short time before landing roles on Glee, these celebrities had some quick romances with their costars when the cameras weren’t rolling.

Below, see which actors admitted to hooking up with their former costars.

Ione Skye and John Cusack

In her newly released memoir Say Everything, Ione Skye revealed that she and her Say Anything costar John Cusack slept together after filming the iconic ’80s movie.

Though she had long said their chemistry stopped at mutual crushes, the actress recently wrote in her book that they hooked up after her divorce from her first husband, Adam Horovitz.

“I had to get it out of my system,” she revealed.

John Stamos and Demi Moore

In John Stamos’ 2023 memoir If You Would Have Told Me, the Full House alum recalled a conversation he and his former General Hospital costar Demi Moore and writer Ali Adler had about their dynamic at the time.

Per E! News, Adler asked the duo “‘Oh my god, Blackie and Jackie! Did you two ever f—?'” referencing their characters on the soap opera in the ’80s.

“We both look at each other, wondering what the other will say,” Stamos wrote, per the outlet. “Demi pauses to think about it. ‘I don’t know if we slept together. I think we fooled around though.'”

Matthew Perry and Julia Roberts

As is the case with many of the after-the-fact romance confessions, Matthew Perry opened up about his relationship with Julia Roberts in his 2022 memoir Friends, Lovers and the Big Terrible Thing.

In the book, Perry — who died in Oct, 2023 from the acute effects of ketamine — shared that when Roberts was in the process of signing on to appear as a guest star on Friends, she said she’d only do so if she was a part of his storyline. The two ended up dating, he explains: “We would already be a couple by the time we started filming the Friends Super Bowl episode.”

“Two months later, I was single,” Perry wrote. “Dating Julia Roberts had been too much for me. I had been constantly certain that she was going to break up with me. Why would she not? I was not enough; I could never be enough; I was broken, bent, unloveable.”

He continued: “So instead of facing the inevitable agony of losing her, I broke up with the beautiful and brilliant Julia Roberts. She might have considered herself slumming it with a TV guy, and TV guy was now breaking up with her. I can’t begin to describe the look of confusion on her face.”

Carrie Fisher and Harrison Ford

Forty years after filming the original Star Wars movie, Carrie Fisher — who famously played Princess Leia — revealed that she and costar Harrison Ford had a three-month affair during production.

She wrote about the relationship in her 2016 book The Princess Diarist, telling PEOPLE, “It was Han and Leia during the week, and Carrie and Harrison during the weekend.”

Lea Michele and Matthew Morrison

Though their romance did not happen during their time on Glee, Lea Michele shared in her book that she and costar Matthew Morrison had dated for a short time before appearing on the Fox series.

“When we shot the pilot, I met Cory Monteith, Kevin McHale, Amber Riley and Chris Colfer for the first time,” she wrote in her book Brunette Ambition, per E! News.” I had worked on Broadway with Jenna Ushkowitz and Matthew Morrison before. Matt had been a friend of mine for years, and in fact we’d actually dated back in the day for a Broadway beat.”

Maureen McCormick and Barry Williams

Decades after starring as step-siblings Marcia and Greg Brady on The Brady Bunch, Williams revealed to Oprah Winfrey in 2013, that he and Maureen McCormick had dated at one point and that he was her first kiss.

“That was in Hawaii while we were filming those episodes. We snuck away, it was a full moon, and walked along the sand. Well, I knew where that was headed. Of course, I set it up,” he shared.

“We couldn’t hold back any longer,” McCormick wrote of the incident in her book, Here’s the Story: Surviving Marcia Brady and Finding My True Voice, per CBS News. “It was our first kiss, and it was long, passionate and deep. It was wonderful, too, though as we continued to kiss and press against each other so closely that we could feel each other’s body heat, a part of me — a tiny part, admittedly — said to myself, `Oh my God! I’m kissing my brother. What am I doing?”‘

Mischa Barton and Ben McKenzie

While appearing on an episode of Call Her Daddy in 2024, Mischa Barton opened up about her off-camera relationship with onscreen boyfriend Ben McKenzie which took place as they filmed The O.C. in the early 2000s.

“It was kind of complicated for me,” she explained.

“It was a lot of my firsts, let’s put it like that,” she added. “And that kind of really separated me, a little, from them, in the sense that I wasn’t out there living on my own in L.A. yet,” she said of her life compared to her older costars (she was 17 when she show premiered in 2003 while McKenzie was 25.)

She shared that her relationship with McKenzie was her “first”: “I had no idea what I was doing.”

Barton said the situation got “tricky” after the two went their separate ways and still had to film together.

“That’s in the very beginning of the show before we’re even halfway through a season, so there was a lot going on there. That show … so much happened in three seasons,” she said of her time on the series.

Michael Cera and Aubrey Plaza

Six years after starring together in 2010’s Scott Pilgrim vs. the World, Aubrey Plaza shared that she and Michael Cera  dated for about “a year and a half.” Years later, Cera went on to share that the two almost got married during a drive through of Las Vegas.

“We almost just spontaneously took a detour and got married,” he told Rolling Stone in 2023. “I think the idea was to then get a divorce right away, so we could call each other ‘my ex-husband’ and ‘my ex-wife’ at like 20.”

Cera married his wife Nadine in 2018 before the pair welcomed their first child together in 2021. The same year, Plaza wed husband, Jeff Baena, who died by suicide on Jan. 3.