In the series, which is based on true events, Sean plays U.S. Attorney General John Mitchell, and Julia plays his wife, Martha. Due to their differing views on the Watergate scandal, the couple scream and fight, and Sean even slaps Julia in one heated scene.
"It wasn't a little kiddie soft-paws slap," Julia revealed on Sirius XM's The Jess Cagle Show. Instead, the actress described it as the type of whack that left her "stunned."
"He gave me such a wallop," she said, before giving herself credit for staying in character. "It's my greatest performance—that I continued to perform and didn't just go, 'Oh my God, that really hurt.' It was astonishing."
So when it came time for her to let him have it, Julia didn't hold back—and Sean noticed. "He told me it seemed to come from the shoulder," she shared, "so I guess I really had a strong response."
Julia noted that the sincerity of their performance wouldn't have been possible if they weren't friends before, explaining, "Sean and I have known each other since I was a teenager, so I knew that I could trust whatever he was gonna do. I was gonna survive."
And while the My Best Friend's Wedding actress hasn't "done a lot of fight scenes," she knew "when you're gonna commit to something like that, you have to really commit to it." So she decided to let Sean set the tone for the scene, recalling that she told him, "You kind have the harder part cuz you, you have to start it. You have to slap me. I just respond."
Fans can see Julia and Sean's heated confrontation when Gaslit premieres April 24 on Starz.