1. It Took DiCaprio and Scorsese Only One Day to Decide to Do It
"We read the script in one day and called each other the next day and said 'Let's do this,'" recalled Leonardo DiCaprio in an interview. He said  William Monahan's script "was so well-written."

2. Nicholson Didn't Want His Role at First
The three-time Oscar winner initially declined the role of Boston gang boss Frank Costello. "I always give a fast no when it's no, and originally there wasn’t a part there," Nicholson told New York Magazine. "I said, 'I'd love to work with you, Marty, I've always wanted to work with you -- and Leo -- but I just can’t do something because I like the idea. I gotta have a part that I’m interested in.'" Scorsese, along with DiCaprio and Damon -- who were already cast -- agreed to expand the part. 

3. Dead Guy Easter Eggs
Scorsese put a subtle "X" in the frame whenever anyone was killed onscreen as an homage to the 1932 version of "Scarface," which is one of his favorite films and first employed the X factor.

4. Sex Scenes Were Invented
Nicholson suggested his character have sex scenes. "These kind of monsters, they don’t usually have a sex life onscreen, so I wanted to bring that to the part," he said in a 2006 interview. "I pushed that side pretty good. He's a mad, bad nut job, so he's evil sexually too." 

5. Robert De Niro Was Initially Cast
The actor, a Scorsese mainstay going back to 1973's "Mean Streets, eventually decided to ditch "The Departed" to direct "The Good Shepard."

6. A First-Time Heavyweight Team-Up
While it was highly publicized at the time, it's still hard to believe the film marked Nicholson and Scorsese's first ever collaboration. Unlike DiCaprio, who has done five Scorsese films, the two haven't worked together since. Hey, there's still time.

7. Brad Pitt Was a Producer
Pitt was initially attached to either one of the two lead roles eventually played by DiCaprio and Damon. Pitt eventually served as a producer on the film, one of the first projects out of his Plan B Entertainment.

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8. Boston Gangster Whitey Bulger Served as Inspiration
Years before Johnny Depp signed on to play Bulger in a biopic (and five years before he was caught hiding out in California), Nicholson used the infamous gangster -- and FBI informant -- as a blueprint for the ruthless Costello.

9. It Got Scorsese His Only Oscar
Scorsese, one of the all-time greatest filmmakers, has won only one Academy Award in his six-decade career -- for directing "The Departed."

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10. Nicholson Went Off Script -- a Lot
"You never never know what to expect from him because he can go off the cuff and just say anything or do anything," DiCaprio said in an interview, recalling Nicholson's many improvisations during filming. "In character, it instills this constant fear in you."

11. It Owes a Huge Debt to Another Movie
Andy Lau's 2002 Hong Kong crime thriller "Infernal Affairs," released four years prior to "The Departed," bears some uncanny similarities to the Oscar-winning Scorsese film. Like "The Departed," it is centered on a police officer who infiltrates organized crime, and also a dirty copy working for the same gang.

“'Infernal Affairs' is a very good example of why I love the Hong Kong Cinema, but 'The Departed' is not a remake of that film," Scorsese has said. "Our film was inspired by 'Infernal Affairs,' because of the nature of the story. However, the world Monahan created is very different from the Hong Kong film."

Said Lau: "Of course I think the version I made is better, but the Hollywood version is pretty good too."

12. There Was a "Basketball Diaries" Brotherhood
DiCaprio said that knowing Wahlberg since their time working together on 1995's "Basketball Diaries" (pictured) made it easier to "rough each other up" in some scenes. "Although he is a big dude," said the actor of Wahlberg. "No joke. As much as I tried to work out and make myself hard-core for this film, that's a solid man."

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13. The Boston Celtics Were Banned
Famous for being a diehard Lakers fan, Jack Nicholson reportedly banned all rival Celtics merchandise during filming.

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14. Nicholson Also Refused to Don a Red Sox cap
"First of all, they wanted me to wear a Red Sox hat," the actor grumbled during an interview with New York Magazine. "But I said, all things being equal, I don’t want to. My Yanks, they came before the ­Lakers, of course." Nicholson is originally from New Jersey.

15. Nicholson Didn't Like Ray Winstone, Apparently
"Me and Jack did not seem to get on too well," Winstone said in 2014. He played Arnold "Frenchy" French, right-hand man to Nicholson's crime boss Costello. "Maybe he was going through a funny time ... Everyone else loves him to death -- I just wanted him to be a great guy. ... We just did not click."

16. It Marked a Departure for Both Actor and Director
"I got to play somebody who was constantly in angst, having 24-hour panic attacks, which is something I never got to do before," said DiCaprio, who argued the film also marked a departure for Scorsese because it involved "intelligence, counter intelligence, the police department, FBI, Irish mobsters in Boston as opposed to Italian guys in New York."

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17. Mark Wahlberg's Boston Roots Helped
"My growing up there has an affect on everything that I do," said Wahlberg. "The only difference was I was playing one of the cops [like the ones] that used to arrest me all the time. You usually become a crook or a cop in Boston -- or a construction worker. There's not much in between."

18. There Has Been Sequel Chatter for Years
Dating back to shortly after the release of "The Departed," reports have swirled that a sequel starring Robert De Niro was in the works. In 2011, screenwriter William Monahan reignited the idea when he described his concept for a followup film in an interview with Collider. So far, nothing has been greenlit, as Scorsese has been kept busy with acclaimed films including "Hugo," "The Wolf of Wall Street," several other producing projects and his upcoming film "Silence," about 17th century Jesuit priests, starring Adam Driver, Liam Neeson and Andrew Garfield.