Josh Hartnett Chose to Be Shirtless in “Trap”'s Climactic Scene: 'I Didn't Want to Cheat the Audience' (Exclusive)

"I wanted it to be as raw and scary and f---ed up as possible," says the actor

Note: This article contains spoilers for Trap, in theaters now.

Josh Hartnett is explaining why he opted to be shirtless for that chilling scene near the end of Trap.

In the thriller, written and directed by M. Night Shyamalan, Hartnett plays Cooper, a suburban dad who leads a double life as a serial killer nicknamed The Butcher.

In one climactic scene, Cooper, who is on the run from the FBI after being exposed, confronts his wife (Alison Pill) while she's home alone. She admits to tipping off authorities about attending the Lady Raven concert, where they staged a trap to catch him.

The tense sequence sees Cooper remove his shirt before trying to kill Pill's character, who, it turns out, poisoned his slice of pie just in time. Hartnett, 46, tells PEOPLE it was his idea to be shirtless for that moment.

"I didn't want to cheat the audience," he says. "I wanted it to be as raw and scary and f---ed up as possible. Knowing that he's OCD, and that he's got these sort of qualities about himself that doesn't want to make a mess, I thought wouldn't it be interesting if over the course of this he just starts taking his shirt off and it gets really visceral."

"Then gouging the guy's eyes, all that monster stuff was really important to me," continues Hartnett, "because I felt like you had to know that this person you've been following, who you may be charmed by, is an actual monster. I think it's important for the movie."

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Cooper, delirious from being drugged, sees dead people in the scene (visions of his late mother), which also features him being tased many times.

"That was the one part we couldn't be accurate on because from a physiological standpoint, I don't think anybody can withstand the amount of tasing that he was getting and still get up," says Hartnett.

"But Night wanted to create something that was really jarring and shocking, and have [Hayley Mills'] character get the final shot," the actor adds. "But no, I was never tased. I've seen videos of people being tased. I would definitely not want to do that!"

Original star of The Parent Trap, Mills, 78, plays an expert FBI profiler in Trap, leading the operation to finally catch The Butcher.

Shyamalan, 53, told PEOPLE at the film's premiere in New York City July 24 of casting Mills, "We've seen the guy hunting a guy for so long in so many movies, so I was thinking, why don't we have a maternal figure hunting these kind of men? Then when Hayley auditioned, I was like, wow, she matches the buoyancy of the movie, that kind of energy and that wink in her eye that she's having fun doing this."

At the same red carpet, Mills said she "didn't expect to ever get a part like this."

"When it happened, I thought they'd made a mistake!" she said. "To work with Night was just a gift. He is a fabulous director. He loves actors. And he writes the characters, he understands them. He knows their thought process. He knows where they're coming from, and if you're not thinking what you're supposed to be thinking, he knows."

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Hartnett says Mills is "the most gregarious human being on the planet." He adds of his costar, "She wants to talk about everything. I loved her. She was there for pretty much the whole time we were shooting. When she was on set we'd just sit and chat."

"And my daughters love her work," adds the father of four. "They've watched Pollyanna a thousand times, and Parent Trap, and they came to the set but for whatever reason they didn't cross paths and they were so angry when they found out they weren't going to meet her. We're going to have to meet up in London or something."

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