Julia Roberts Talks Sean Penn's Crazy Transformation for Gaslit & Raising Her Kids with 'Conviction'

Michael Kovac/Getty Julia Roberts and Sean Penn in 2020

Sean Penn's transformation for Gaslit was so involved that his costar Julia Roberts says she sometimes wouldn't see him "for weeks."

Roberts, 54, plays Martha Mitchell, the first person to publicly accuse President Richard Nixon of being involved in Watergate, in the upcoming Starz series, while Penn, 61, plays her husband Nixon's Attorney General John Mitchell.

Of Penn's transformation, Roberts said on Wednesday's E! Daily Pop, "He would get to work before me. He would be getting his makeup done."

"When I got to work, he would already look like John Mitchell, and then I'd be gone before he was transformed back into Sean Penn," she added.

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human-interest stories.

starz Sean Penn and Julia Roberts in Gaslit

RELATED: Julia Roberts Stars as the Fearless Martha Mitchell in the First Trailer for Gaslit

The actress also spoke to E! about raising her three children with husband Danny Moder: son Henry, 14, and 17-year-old twins Phinnaeus and Hazel.

"I think it's our personal responsibility to advocate for ourselves, and I think that my husband and I both are people of strong convictions and opinions and compassions and I think that we try to instill that in our kids as much by example as by preaching," Roberts said.

The Academy Award winner went on to note that her children are "coming to age within" what she considers "an interesting society and culture."

"I think that ... they bear a huge responsibility — more than I think I did at their age — to speak their minds and speak up for themselves and advocate for the things that they believe in," Roberts said.

Gaslit is based on the first season of the critically acclaimed Slow Burn podcast. The series is a modern take on Watergate that focuses on the untold story of Martha Mitchell, whose accusations of Nixon cause both the presidency and her personal life to quickly unravel.

As attorney general, John Mitchell is Nixon's most trusted advisor and best friend. Temperamental and ruthless — yet hopelessly in love with his famously outspoken wife — he'll be forced to choose between Martha and the president.

Roberts told E! of her character, "She's so big and she's so clearly this outspoken and opinionated person, but she also was very devoted to her husband and her family."

"She was very shy. She really didn't like public speaking," the actress added. "There's all these paradoxes about her that were fun to get to play with."

Gaslit premieres April 24 on Starz.