Rose Byrne has gotten to do what few would dare try! The Immortal Life of Henrietta Lacks star played journalist Rebecca in the upcoming HBO film and in one particularly tense scene, she had to tell Oprah Winfrey’s character, Deborah, to “shut the **** up.”

“It was intense! It was a long day, but we wanted to do a good job and we were g-ed up to do it,” she told ET’s Jennifer Peros at the film’s premiere on Tuesday. “It was one of those strange surreal moments in your life but that's acting you know? I think I said that to Glenn Close a few times as well so I'm clearly cornering a market, telling legendary woman to eff off.”

WATCH: Reese Witherspoon Dishes on What It's Like to Make a Movie With Oprah Winfrey: 'She'd Do the Voice!'

Winfrey also admitted to being intimidated by Byrne’s lengthy acting credits, which the Australian actress was surprised by.

“Yeah, very strange. She's incredibly humble for someone of her, being a living legend really,” Byrne said. “She had a huge undertaking and she was nervous admittedly, as was I. It's a very important story and a big story we tried to tell in a short amount of time too so it was ambitious.”

Byrne was also admittedly intimidated by her A-list co-star, saying, “Of course, someone of that profile, and [me] playing very intense roles, a journalist trying to really get in her space, you know so, and with somebody whose profile everybody wants a piece of you but we leaned heavily on George Wolf our director, he really navigated us through everything.”

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Even with the allure of her legendary co-star, Byrne had a hard time returning to work so soon after the birth of her son, Rocco.

“It was my first job since having my son. I didn't really particularly want to go back to work but I couldn't say no, I was like, 'I can't say no to this project,'” she said of the inspirational story. “It's exceptional and to be in service of this story is my honor.”

Byrne also revealed that becoming a mom has changed her mindset when it comes to taking on new roles.

“Because you want to spend time with them, but I love to work and I want to work as well, so like any working parent, man or woman, you figure it out,” she said. “I don't know, I’m figuring it out every day. I'm just as naive about that as the next. I'm still trying to figure it all out.”

The Immortal Life of Hernrietta Lacks premieres on April 22 at 8 p.m. ET on HBO.