Jessica Chastain told RTÉ Entertainment that she was "blown away" by how "up for anything" her co-star James McAvoy was when filming IT Chapter 2.
"I was really blown away by how James was really up for anything," Chastain told RTÉ Entertainment.
"He was doing rolls and flips and slides and he was full on James McAvoy action star. What we'd done before, I'd never seen that side to him."
Despite being known for their on-set pranks on past projects, the creepy set did not lend itself to any messing around and she was certain director Andy Muschietti would not try to "traumatise" them.
"I didn't think Andy would ever try to traumatise us on set, thank goodness, so I didn't think anything on set would make me feel unsafe," she said.
IT Chapter 2 hits cinemas on September 6.
Watch our interview with director Andy Muschietti: