Julia Roberts and Sean Penn on Taking Their Close Friendship to a Working Relationship on 'Gaslit' (Exclusive)

The longtime friends were excited to work together for the first time.

Julia Roberts and Sean Penn taking their real-life friendship to a working relationship couldn't have gone better. ET spoke to the actors at the premiere of their new show, Gaslit, in New York City on Monday, and they weren't shy about praising each other.

"You role the dice when you act with someone who was your friend before, because you don’t know what that process is like or what it's gonna be like," Roberts, 54, explained.

"There is that personal part where it's an adored friend and an adored actress and talent," Penn noted. "I think to work with someone... who raises your game in terms of material, but also in terms of the professional commitment to that material, it is that kind of job working with Julia."

Penn, 61, added that, because of Roberts, during filming, "You don’t think, 'Should I be doing something else?' No. You feel like you're where you're supposed to be."

In the Starz series, Penn and Roberts play John and Martha Mitchell, Richard Nixon's attorney general and his wife, the latter of whom was the first person to publicly sound the alarm on the then-president's involvement in Watergate.

For Penn, the role meant a physical transformation, one he credits to Kazu Hiro, his prosthetic hair and makeup designer.

"This makeup is taken to da Vinci level kind of artistry that is now part of actors tools boxes if they chose to use it," Penn said. "It takes you a long way."

While Roberts was equally complimentary of Hiro's work, she praised Penn for how he worked to embody the character. 

"He has to wear [it] and he has to make it real," she said. "He was so patient with it. He was never hot or itchy or [saying,] 'Get this off me.' He was always present."

Gaslit will premiere April 24 on Starz.