The Sixth Sense” was almost a serial killer film inspired by “The Silence of the Lambs.” In the original draft of the thriller by director and writer M. Night Shyamalan, Bruce Willis’ character was a crime photographer (instead of a therapist) with a son who experienced visions of the victims. Ten drafts later, Shyamalan morphed the script into what we know today: a psychological drama with a monumental twist ending that would launch the career of a young director with comparisons to Spielberg and Hitchcock.

“The Sixth Sense” opened on Aug. 6, 1999 as an under-the-radar summer release with little fanfare and low expectations. But within only two weeks, the film made back its production budget of $40 million and received largely positive reviews. It ultimately earned a staggering $672 million worldwide, and it became the second-highest grossing movie of 1999 at the domestic box office, beating out blockbuster tentpoles such as “Toy Story 2,” “Austin Powers: The Spy Who Shagged Me” and “The Matrix.” It was only behind “Stars Wars: Episode 1 — the Phantom Menace.”

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Haley Joel Osment, who plays the young boy who utters the words “I see dead people,” told Variety that the cast knew intuitively that the script was “something really special,” and they were right: “The Sixth Sense” earned six Oscar nominations including best picture, best director, best supporting actress for Toni Collette and best supporting actor for then 10-year-old Osment.

For those who haven’t already seen the horror classic, the story follows a boy named Cole Sear (Osment) who can see the dead. His mother (Toni Collette) hires a therapist named Dr. Malcolm Crowe (Bruce Willis) to assess Cole’s social and emotional problems. There are also memorable supporting roles for newcomer Mischa Barton, as a child ghost who had been poisoned by her mom, and Donnie Wahlberg, portraying a vengeful ex-patient who corners Willis in his home early in the film.

Now 20 years later, the cast speaks with Variety about the making of the Hollywood staple and why it was such a defining film.

M. Night Shyamalan, who is now 48, spent nearly a year on the script, not sure where the story should go or what it should be. The movie arrived in theaters on his 29th birthday.

Shyamalan: Oh god, it was a long process of writing it.

The film landed at Disney’s Buena Vista Pictures, with Shyamalan set to direct his own script. Wahlberg was so committed to the character that he stopped showering. Shyamalan was worried that Toni Collette wouldn’t get the studio’s approval, because she had a shaved head. But there was one young actor who blew him away.

Shyamalan: When Haley came in and read, I knew he was the kid the second he read it. I felt overwhelmed with that knowledge. And so once I knew him, I had seen “Muriel’s Wedding” and Toni came in and her head was shaved, and I forget if it was for fun or if it was for a movie. She did such a beautiful job. I didn’t want to show the video to the studio for fear that they would be concerned with her appearance, and I said “I want to cast the woman from ‘Muriel’s Wedding'” and then Bruce backed me and said “Oh, I love ‘Muriel’s Wedding,’” so we kind of got it without the studio seeing the audition, and I was so lucky. Toni is actually wearing a wig throughout all of “The Sixth Sense,” and I think it’s a wig from “Velvet Goldmine.” We didn’t even have the wig!

And then, of course, there was Donnie [Wahlberg], who was kind of a lightning bolt for us. And, again, he, like Haley, brought a serious kind of commitment to his role, and even though it was one scene, he really, really lived in it, he lost tons of weight and didn’t bathe. He really went for it, and it comes off.

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Haley Joel Osment (Cole Sear): I’d never read a script like that. I had done some films, and I had some network sitcoms. This script was a whole other ball game. In terms of the whole process starting with the audition, it was kind of the first time at around age 10 that I think I started to get a real understanding of what acting was and everything and getting used to being on sets, learning your lines and the job part of it.

For many of the interior scenes like in the Sear’s house (think back to Collette getting freaked out in their kitchen after seeing all of her cupboards opened out of nowhere), the cast and crew shot in a creepy Philadelphia convention center with makeup and the kids’ tutors nestled in the caverns of the eerie building.

Osment: We were shooting at the old Philadelphia convention center which I believe has since been demolished. Big, empty marble hallways, kind of a “[The] Shining” feel around the set itself. I believe JFK did a speech there during a campaign in 1960. It was very old and ornate and had these cavernous, marble stairways that went really deep down. Yes, it really had a creepy feeling.

Mischa Barton (Kyra Collins): I don’t know if it’s just kids who make up that’s everything’s haunted, but I’m pretty sure that place was haunted. Like, actually haunted. I don’t really want to speak for Night, but I really think he kind of enjoyed it because it had this creepy air to it that made it even more fun.

Shyamalan: It was a very, very big building and most of it was empty, and so I can imagine for children that are running around there, it did feel very occupied spiritually. A lot of corridors and empty, not great lighting, flickering bathrooms and things like that.

Barton, then 13, and Osment, then 10, were fine — and their parents weren’t fazed by it.

Osment: I don’t remember them being concerned about it. It’s not a particularly graphic film.

Barton: It wasn’t a very big role obviously, but I got to spend a lot more time [on set] and commute from New York to Pennsylvania, and we would take the train. My mom never really shied away and neither did I from quite heavy material. While we were filming, it was kind of funny. It didn’t really have this heavy connotation that you might expect.

Willis spent nights as a DJ for parties with the adult members of the cast, and he gave Night his first-ever hangover. Barton, in her creepy makeup, sometimes ran away from tutors to sneak around the “haunted” convention center and would play ball with Osment and the other kids.

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Barton: Something that I always remember is that Haley was always playing. He would have a tennis ball that he would bounce off the wall just to pass the time. I always wanted to get in on the game. As a girl it was like, “I kinda want to come play with you boys.”

Osment: I remember we played racquetball in one of the big marble hallways when [Barton] was in her scary makeup.

Shyamalan: Bruce definitely introduced me to the notion of partying and letting loose. Back in those days, he was a big DJ and the parties were super fun. He definitely gave me my first hangover. He’d always keep giving me shots that he kept calling “candy”: “Here’s candy, here’s candy.” And then the next morning I could not get off the sofa. And I didn’t know what this throbbing pain in my head was. He was just laughing his butt off.

When thinking back to the scariest scene in “The Sixth Sense,” most will recall Barton writhing under a bed, trying to get Osment to realize her mother killed her. In white makeup, she’s almost unrecognizable from this role to her next one on “The O.C.”

Barton: We would make a mixture to put in my mouth. They gave me options between cereals and bananas. And then, my mother [in the film] has Munchausen [syndrome] where you’re poisoning your child, so that’s a really intense thing, but creepy is part of my vernacular. It doesn’t really bother me that much. I’ve always been fascinated by gothic and dark things. So, it wasn’t like I was scared. It was more just really interesting subject matter. I’ve always felt like there’s a real need to do the character justice. Everybody just asks, “Was it traumatizing?” It really wasn’t. I was already 13 and I could handle it.

Later on, Osment finally tells his mother that he can talk to his dead grandmother. “Do I make her proud?” Cole’s mom asks. The ghost’s response? “Every day.”

Osment: That was one where we actually walked away from the day going, “Gosh, I really hope we got that.” We were feeling kind of nervous about it because it had been so hard.

Shyamalan: I felt I had all the pieces, but I was not 100 percent sure, because we didn’t do [the scene] in full takes that I felt comfortable with. We had to move on. I walked away, and I was talking to somebody else and then the AD came over and said “Hey, they’re both [Collette and Osment] really upset.” And I went back to the car and they were both crying and they said, “Are you sure we got it? Are you sure we got it?” And I go, “We got it,” and I gave them reassurance. I said, “I’m pretty sure we got it.” And, we did get it.

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The most important moment in “The Sixth Sense” involves a line of dialogue that played in the trailer. It’s of Cole in a freezing cold bed saying the line, “I see dead people.” Shyamalan decided not to use CGI to facilitate the image of his breath; instead, he put Osment in a a real-life ice box.

Osment: This was in the early days of CGI, and we didn’t use CGI for the scenes where it was cold, and you could see our breath. What they did was they would drape this huge plastic sheeting over the sets and then pump in freezing cold air so that it would be below freezing, and you could see our breath. There was a limited time that we could be in there because it was so cold and most of the scenes I’m in my underwear or something. It’s a tough environment, but it’s great when you’re in a scene where you’re supposed to be frightened and shivering and it really is that cold. You can actually use real elements in those situations.

Shyamalan: CGI at that time was not perfected to the place where I felt comfortable that it could do breaths. So, we built a cold room. [Osment] wasn’t acting, it was cold, and you could see the physicality on his skin and the way he’s shivering. And even now, with CGI, I might do it the same way because of what it makes the actors do.

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While filming, the cast wasn’t totally aware of how terrifying the film would be. Mostly, they thought it was an emotional drama about authentic characters grappling with communication issues with their loved ones: dead or alive.

Osment: The first time I remember seeing a lot of it other than dailies was at Disney’s lot in Burbank in the Seven Dwarves building where they had some screening rooms. I went there with my whole family except for my younger sister who was too young. A friend of mine came, and I remember him getting really scared by it. That was the first time I was like, “Oh, wow, this is what this movie does to people.”

Barton: The first time we realized how intense and scary it was was when I went in to do ADR, and I actually had my little sister in the room because we went into this studio in New York to do the ADR and she just, like, literally flipped. You know how you’re not supposed to make any noise in ADR? She screamed at the top of her lungs. My mom was like, “Oh, my god.” She had to take her outside. My sister’s like, “That’s the scariest thing I’ve seen.” I never went back to school as just a regular kid anymore. All these kids went to the movie theater and they were like “Is that you?” The whole school treated me differently. I don’t think the teachers knew what to do with me.

Shyamalan, of course, would go on to have a prolific career in Hollywood, with “Unbreakable,” “Signs” and “Split,” which earned back his credibility in Hollywood after a cold streak. However, his tendency to push for twist endings has divided some critics.

Shyamalan: “The Sixth Sense” was the movie that didn’t have the legacy to deal with. It didn’t have my name to deal with. So, it would be interesting if “The Sixth Sense” was the third movie or the fourth movie and how that would’ve changed the audience’s relationship to the film. Could you even watch the movie? Or would you from the first moment in the movie go, “Oh, I know what’s happening.” It’s a really interesting thing. That movie created a relationship with my name and then the name itself now has a framing for all the rest of its cousins. It’s the one movie that got to live without my name.