Sydney Sweeney Says It Was 'Terrifying' to Film in Real Catacombs for “Immaculate” (Exclusive)

"You only could be down there for so many minutes because of the air quality," the actress says of filming 'Immaculate' inside real underground passageways

Sydney Sweeney got familiar with fake blood for her twisted new movie Immaculate.

In the film, the actress plays Cecilia, a devout American nun who moves to a convent in Italy, quickly uncovering a sinister side of the religious community.

Sweeney, 26, tells PEOPLE that wearing the nun costume was "comfy," except "the habit was probably the most uncomfortable piece of the wardrobe. It would crush your ears in a weird way. But everything else was great."

Then, however, came the fake blood. "It is very sticky, and it's usually cold," says the actress. "The worst is getting it put on you, but then after a while you're fine. You forget that you have it on."

"But then the removal process is extensive," says Sweeney, also a producer on the project.

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Related: Sydney Sweeney Says She and Anyone But You Costar Glen Powell Are 'Dreaming Up' Ideas for Sequel (Exclusive)

Director Michael Mohan estimates they went through "several gallons, for sure," of prop blood and says Sweeney "has this remarkable ability to just go to these dark places at the drop of a hat."

Sweeney — who says, in her own life, her fears are "clowns and needles" — explains it's not difficult for her to shift out of that dark mind set between takes.

"It's an easy on-and-off switch," says Sweeney. "I learned from an early age that it's important to separate myself from my characters as much as possible. Because I did that, I can just jump in and out of what I'm doing."

Still, filming part of Immaculate inside real underground passageways was a "creepy" day of work.

"Being able to film in actual catacombs, that was really terrifying and haunting and a technical experience that we had because you only could be down there for so many minutes because of the air quality, and only so many crew members could come down with you," she explains.

Adds Sweeney, "It was just creepy, too, seeing everything that was down there."

The production went for realism, which resulted in a gory final product, so much so that director Mohan is "very shocked we got an R-rating" with no revisions.

"The MO was never, 'We're making a gore film.' It was more that because the film's not supernatural, we wanted it to just feel as real as possible," says Mohan.

"Also, we're not really lingering on those shots very long, but they really do lodge into your brain in a way that— you just can't unsee some of the imagery in this film."

Immaculate is in theaters Friday.

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