The Oscar winners lead the Watergate drama in a portrait of a marriage that crumbled over scandal.

For Julia Roberts, Gaslit wasn't the story of a notorious political scandal, but rather the story of a marriage, and how loyalty to a president over a partner — an affair with Richard Nixon, if you will — tore that marriage apart.

The Oscar-winning actress leads the Starz Watergate drama's ensemble cast as Martha Mitchell, whose union to Nixon's attorney general and campaign manager John Mitchell — played by Sean Penn, with the help of some heavy prosthetic makeup — came undone as the scandal unraveled. (One day, the real Martha Mitchell told PEOPLE in 1974, John suddenly "asked if I had dresses to wear to his trial. I just looked at him and ran from the room and cried and cried and cried. John Mitchell, who stood for law and order!")

"So much of her is motivated and driven by her relationship with John Mitchell," Roberts says of Martha. "I do think they were wildly in love, and I think they had this great, passionate relationship. And in the end, it was her downfall, and I think that without him she ceased to feel like she knew who she was."

Green Room; Martha Mitchell (Julia Roberts); bts
Julia Roberts as Martha Mitchell in 'Gaslit'
| Credit: Starz

Roberts immersed herself in research to play the once-remarkably famous socialite, an Arkansas native who was dubbed the "Mouth of the South" for her outspoken habits. "One of the great things about portraying this person was there was a lot of source material, a lot of archival footage, a lot of photographs," says the actress. "I could see her being interviewed, and the ways that she talked, the way that she walked, the way that she dressed. There was so much that I could excavate from all those things to put together my interpretation of her."

Her devoted approach impressed even creator Robbie Pickering, who had named Roberts his "top choice" for the role of Martha. "She's just such a huge star that you're constantly concerned: How deep is she really going to go?" the showrunner says. "Because if Julia doesn't want to do anything, she don't have to do anything. She can sit there and do nothing, and it's like, you are just lucky to have Julia in your show."

Luckily, "she went so deep with this role," Pickering says. "You're getting the romantic comedy, Notting Hill, My Best Friend's Wedding, Pretty Woman Julia Roberts. You're getting a thriller, 'female in trouble,' Sleeping With the Enemy Julia Roberts. But you're also getting this thing I don't think people have really seen from Julia, which is this messy, complicated marriage between her and Sean. It brings her to this very dark, raw place by the end of it."

But while Pickering says it was surprisingly easy to bring Roberts on board ("I never in a million years thought she'd say yes," he admits), she did have one condition: She wanted Penn cast as her husband. "They have a deep friendship, and they'd always been searching for a role to do together," Pickering explains.

Sean Penn as John Mitchell in 'Gaslit'
| Credit: Hilary Bronwyn Gayle/Starz

"Sean Penn is just the best in the game," says Roberts, who's also an executive producer on Gaslit. "And he took it on with such predictable vigor. To watch him go through the transformation and so fully inhabit John Mitchell, and for us to talk about the Mitchells and their relationship, was a thrill for me."

If Pickering also had reservations about Penn, they vanished quickly after he met with the Oscar-winning actor. "The thing I was concerned about with Sean was, I wasn't writing a political diatribe. I really wanted to focus on the relationships and the human beings at the center of the story," he says. Meeting with Penn over dinner, the writer thought he'd want to talk politics the entire time.

"He didn't," Pickering says. "He just discussed the marriage between these two people. He was bringing stuff from his parents' marriage, from his marriage, and he was just concerned about the reality of this guy as a person."

That aspect was key to the actors' partnership, Roberts says: "Sean and I were both interested in how to excavate the little details of their relationship so that you could appreciate that these two people really did love each other. It was almost like there were two of each of us: our characters when they were together, and then our characters when they were with other people. It was pretty fascinating, the things he accomplished. I was really in awe all the time."

"I think the most underrated aspect of any great Sean Penn performance is his vulnerability and his humanity," adds Pickering. "I was fortunate that Sean Penn just really wanted to show the warmth and humanity of this guy, as well as the evil of him."

Sean Penn and Julia Roberts in 'Gaslit'
| Credit: Hilary Bronwyn Gayle/Starz

Of course, that "evil" won out in the end: The infamous Nixon tapes revealed that John Mitchell was involved in both the planning and cover-up of the Watergate break-in, and he ultimately served 19 months in prison. The Mitchells' marriage collapsed as a result; they separated in 1973, and never saw each other again. Martha died of a rare form of cancer in 1976.

"It's so ultimately sad, the way politics and corruption really destroyed their marriage," Roberts says. "I think it's all sort of connected. Maybe if he had chosen her over Nixon, her life would have been very different."

Gaslit premieres Sunday at 8 p.m. ET/PT on Starz.

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