Shooting “Underwater” was no easy feat for Kristen Stewart.

Stewart and her co-stars spent the majority of their time filming the sci-fi flick in the water wearing heavy 100 lb. scuba suits — a challenge which none of them were prepared for.

“It was so scary and so horrific. It wasn’t fun scary. It was f–ed up scary,” Stewart said with a laugh during a Q&A moderated by “It” director Andrés Muschietti. The cast gathered for the panel Tuesday after a special fan screening at Alamo Drafthouse in Downtown Los Angeles.

It was so terrifying, in fact, that Stewart (who is “scared of swimming”) skipped prep for it. On the other hand, her co-stars Jessica Henwick and John Gallagher Jr. took scuba diving lessons when they arrived in New Orleans to shoot the project.

“Kristen was like, ‘Yeah, I don’t want to learn how to do this. Just put me in [the suit] on the day. I don’t care. It’s going to be so awful,’” director William Eubank recalled.

“Underwater” follows a crew of researchers (including Mamoudou Athie, T.J Miller and Vincent Cassel) stationed on an underwater research facility who get trapped in a flooding structure after a devastating earthquake. The only way to survive is to walk across the ocean floor to a distant abandoned rig, and as they make their way, they soon realize that they are being hunted by monstrous alien-like sea predators that are determined to kill them.

Since they had to shoot in subterranean environments, Stewart said she feared the possibility of using faulty suits or defunct submarines.

“I was just scared,” she said. “I was claustrophobic and I couldn’t deal with that. But this was a f—ing hell hole for a million other reasons.”

Henwick and Gallagher Jr. echoed Stewart’s sentiments, saying none of the cast had any idea shooting the film would be so challenging.

“When I signed read this, I didn’t think it was going to be a physically demanding,” Henwick said. “I thought, ‘This’ll be easy. Just put me in a suit. We’ll be walking around pretending to be under water.’ No. It was the hardest film I’ve done by far.”

Gallagher Jr. said he was equally clueless until he touched down in New Orleans.

“The suits were so hard to move around in that like I don’t remember any acting choice,” Gallagher Jr. chimed in. “And I remember honestly being like, ‘Okay, if Kristen’s scared, then I can be scared too.’ Then we can just kind of go through this.”

But Stewart said ultimately those obstacles, however terrifying, drew her to the film.

“We’re joking about how hard it was to make the movie, but I am attracted to things that are difficult. The only reason to do something is because it scares you.”

“Underwater” is in theaters on Jan. 10.

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