Get ready to be blown away.
During filming, Toni Colette always woke up in the middle of the night and always to a repeating number on her clock like 1:11, 3:33 or 4:44... Creepy! (source)
Bruce Willis turned down the role of Sam Wheat in Ghost because he thought playing a ghost would be detrimental to his career. He only made The Sixth Sense because it was one of two movies he owed Disney after causing another production, The Broadway Bawler, to be shut down when he fired the director. He was paid half of his usual salary at the time but in the end The Sixth Sense was a commercial success and is considered one of his best performances (source).
The color red is intentionally absent from most of the film, but it is used in a few shots for showing something in the real world that has been tainted by the other world, such as the church door, Cole's sweater at the birthday party, Kyra's mother's dress at the wake, and the shawl wrapped about sleeping Anna (source).
Haley Joel Osment got the role of Cole after turning up to the audition wearing a tie. Director M. Night Shyamalan was also very impressed when he asked Haley if he'd read his part in advance and Osment said he read it three times. Shyamalan clarified, "Wow, you read your part three times?" To which Osment replied, "No, I read the script three times" (source).
Toni Colette was so moved by the emotional arc of the story that she didn't even realize it was a horror movie until she saw the premiere (source).
The line "I see dead people" was voted as the #44 movie quote (out of 100) by the American Film Institute (source).
Throughout the film Malcolm only wears variations of the clothes he was wearing or things he touched the evening before he died, including his overcoat and the different layers of his suit (source).
The movie was inspired by an episode of Are You Afraid of the Dark? directed by David Winning. In the episode lead characters are ignored by everyone else and don't realize they are dead until the final moment (source).
If you watch the movie on the Vista Series DVD and wait long enough on the main menu screen, a discordant note will play as a ghost walks past the breakfast table (source).
M. Night Shyamalan left a few clues about Malcolm's situation throughout the movie, though most people don't pick up on them 'til the second watch. When Cole and Malcolm are entering Kyra's bedroom at her wake, Malcolm is standing directly behind Cole; however, when the camera cuts to the shadows on the floor as the door opens, we only see Cole's shadow. Also, on the doorknob, we can see the reflection of Cole's face but not Malcolm's face. Throughout the movie, Malcolm never moves any objects, and he does not interact with anyone but Cole. For example, he never opens a door, the chair he sits in in the restaurant with his wife is already pulled out and when he reaches for the cheque, his wife just beats him to it (source).
Osment later revealed that filming the movie was a lot less scary than watching it saying, "There is something desensitizing (not necessarily in a negative way) about seeing the whole process of making a horror movie that prevents you from being too scared going forward." To this day he still doesn't get too bothered by horror movies (source).
The Sixth Sense was rented by 80 million people in 2000 making it the year's top rated VCR and DVD rental (source).
The Sixth Sense received six Academy Award nominations: Best Picture, Best Director for M. Night Shyamalan, Best Supporting Actor for Haley Joel Osment, Best Supporting Actress for Toni Colette, Best Original Screenplay, and Best Editing. It is one of only five horror films to receive an Academy Award nomination for Best Picture after The Exorcist, Jaws, The Silence of the Lambs, and Black Swan. The Sixth Sense didn't win any of it's nominations (source).
Osment and Donnie Wahlberg's character, who can also see dead people, both had white hair on their heads. Shyamalan wanted all the "spirit spotters" would have a distinct yet similar physical attribute (source).
The president of Walt Disney Studios at the time, David Vogel, didn't consult with his supervisors before paying $2.25 million for the rights to the script and signing on Shyamalan as director (source).
The movie was so much of a sleeper hit that in Entertainment Weekly's Summer Movie Preview of 134 films to see in summer 1999, The Sixth Sense wasn't mentioned once (source).
In the scene when Cole says the famous line, "I see dead people," the camera does a closeup on Bruce Willis's face. Producer Frank Marshall was worried that might have given the game away. It implied that Malcolm was a dead person. Fortunately, none of the audiences in the test screenings or afterwards picked up on it (source).
Many members of M. Night Shyamalan's family are doctors, which is the reason why he typically cameos as a doctor in his own movies. In this movie plays Dr. Hill, the doctor who examines Cole after he has an "accident" at a birthday party. Unfortunately M. Night thought his own acting was so bad that he cut out most of his scene (source).
Bruce Willis is actually left-handed but learned to write write and draw with his right hand so that viewers wouldn't notice that his wedding ring was missing (source).
If you watch the movie carefully, you'll note that the only one who actually ever acknowledges Malcolm's presence is Cole. For the rest of the movie, you only assume that everyone sees or acknowledges Malcolm simply because of the way the scenes are shot. When a scene starts by showing Malcolm sitting in a room with Cole's mother, we as a movie-going public have been trained to assume that means they have been talking. However, we never actually see that happen (source).