The new horror is out later this year, following the main characters of the first movie; both as children and years later as adults. Chastain plays the older Beverly Marsh, and has revealed a spoiler about Chapter 2 that indicates the sequel will be every bit as disturbing as its record-breaking predecessor.
While appearing on The Tonight Show with Jimmy Fallon, she explained that one scene will have more blood than any other moment in horror movie history.
“It’s scary, man,” she said.
“I love the scary stuff, but it’s more fun to be the scary person than the one getting scared.”
The star added: “I’m gonna say something and I think I’m gonna be in trouble, but I’m gonna do it!
“It might be a spoiler, but in the movie there’s a scene that someone said on set that it’s the most blood that’s ever been in a horror film, in a scene.
“And I’ll tell you the next day I was pulling blood out of my eyeballs – fake blood!”
Joining Chastain in the adult cast will be James McAvoy as Bill, Jay Ryan as Ben, Bill Hader as Richie, Isaiah Mustafa as Mike, James Ransone as Eddie and Andy Bean as Stan.
Bill Skarsgard will reprise the role of It, aka Pennywise.
It was a huge success back in 2017; amassing a hefty $700.4 million at the global box office against a budget of around $35 million.
Andy Muschietti is back to helm the follow-up, with his youngest actors all also reprising their roles.
Filming took place between June and October last year, with cast members posting tantalising updates on social media.
It: Chapter 2 is out on September 6.