Judy Greer Was ‘Starstruck’ by Mark Hamill on Set of ‘The Long Walk’ (Exclusive)

Judy Greer knows Hollywood royalty when she sees it.

Speaking with PEOPLE about her new movie The Long Walk, the actress opened up about working with the talented cast that included lead Cooper Hoffman and Star Wars legend Mark Hamill.

Asked what she and her castmates did to unwind while not filming the harrowing scenes that the thriller is chock full of, Greer, 50, admits, “I was always a little starstruck by Mark Hamill, so whenever he was around me, I had a hard time unwinding.”

On the flip side, she and Cooper — who had his breakout role in 2021’s Licorice Pizza — felt like they’d always known each other when they met for the film, in which they play mother and son.

“Cooper and I, on minute one, immediately had great chemistry and loved each other and started laughing right away,” Greer says. “Even though the scenes were not funny, we were just having so much fun on set, so that was really easy.”

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“I feel like I’ve known him my whole life,” she continues of Cooper, 22. “And technically, I probably could have known him his whole life — I didn’t, but I feel like I have. So that made it simple.”

Cooper — son of the late Philip Seymour Hoffman — plays Ray Garraty, a teenager in a grueling competition to be the last one standing in a group of boys looking to either hit it rich or die trying at the hands of the villainous Major (Hamill, 73), who enforces the totalitarian regime’s cruel rituals.

Greer plays Ginny Garraty, a character who has already seen tragedy in her family but who still supports her son even as she can almost feel him slipping through her fingers in real time.

For a film of this type, Greer tells PEOPLE, “They’re so hard emotionally that you have nothing but to release the pressure valve, so they do tend to strangely be the more fun movies to shoot.”

“It’s interesting, because while obviously Stephen King is the father of horror … this movie is horror, but it’s psychological to me, and so sometimes that is even harder emotionally,” she explains.

Directed by Francis Lawrence and adapted from King’s 1979 dystopian novel of the same name, The Long Walk costars David Jonsson, Ben Wang, Charlie Plummer, Garrett Wareing, Tut Nyuot and other young stars participating in the deadly walking competition alongside Cooper’s Ray.

Recalling the biggest challenges of shooting, which saw the young actors walking nearly 400 miles, Greer says, “It was really hot — it was summertime in Winnipeg. Those poor kids … they were walking so much all day, every day. Someone was always handing out water and Gatorade.”

“I wasn’t there for all of it, obviously, but my hats off to those actors, because they really did it,” she adds. “They really had to be outside walking all the time; that’s not faked.”

The Long Walk is in theaters now.