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It's now been 40 years since Francis Ford Coppola gave us one of the greatest and most mind-bending war movies of all time. "Apocalypse Now" has lost none of its power over the decades. So strap on your helmet and head up-river to learn some interesting trivia about the background and infamously troubled production of "Apocalypse Now."

1. Writer John Milius listened exclusively to music by The Doors and Richard Wagner while he worked on the screenplay. Milius said he believed The Doors to be "the music of war," a fact which greatly upset the members of the band.

2. The majority of the dialogue had to be re-recorded during post-production, as the jungle environments and heavy background noise made much of the original dialogue impossible to use.

3. Coppola originally offered the role of Captain Willard to "The Godfather" star Al Pacino. Pacino declined, telling Coppola he had no interest in spending months shooting in a swamp.

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4. Pacino was also one of several actors Coppola considered as a potential replacement for Marlon Brando, who repeatedly threatened to quit.

5.Harvey Keitel was originally cast as Willard and was fired after two weeks of filming. At least one shot of Keitel's Willard made it into the final version of the film.

6.Martin Sheen largely improvised the scene where Willard has a meltdown in his hotel room. Sheen was heavily intoxicated and actually cut his hand when he punched the mirror.

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7. Sheen's brother Joe Estevez plays an essential but uncredited role in the movie. Estevez served as a stand-in for his brother while Sheen was recuperating from a heart attack, and also provided the voice-over work for a large portion of Captain Willard's narration.

8. Coppola opted to frame Colonel Kurtz in shadow for most of his scenes, mainly to hide the fact that Brando arrived on-set extremely overweight.

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9. The friction between Brando and Coppola became so great that Assistant Director Jerry Ziesmer eventually took over filming for Brando's scenes.

10.Harrison Ford specifically chose his character's name, "G. Lucas," in tribute to "Star Wars" and "American Graffiti" director George Lucas. Lucas was once slated to direct "Apocalypse Now" himself, which he envisioned as a faux-documentary.

11. The film's famously disaster-ridden production is chronicled in the 1991 documentary "Hearts of Darkness: A Filmmaker's Apocalypse." The production went so far over budget that Coppola was forced to mortgage his house and winery in order to finish filming.

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12. Widespread drug abuse was another major problem during filming. Dennis Hopper reportedly caused a teenage Laurence Fishburne to become addicted to heroin.

13. Because the film includes no opening title card or credits, Coppola needed to include the shot of the graffiti tag "Our Motto: Apocalypse Now" in order to ensure the film could be copyrighted.

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14. The American Humane Association slammed the film with an "Unacceptable" rating after it was discovered the scene where the water buffalo is slaughtered was actually real.

15. Coppola repeatedly struggled with his vision for the ending of the movie. Originally, the screenplay ended with Kurtz convincing Willard to join him and both men dying in a military airstrike, but Coppola opted for a less depressing finale.

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16. There are several alternate versions of the film in existence. 2001's "Apocalypse Now Redux" adds 49 minutes of deleted footage, including the lengthy sequence where Willard and his crew encounter the French plantation owners. A bootleg workprint cut includes even more deleted footage, such as a death scene for Hopper's character.

17. 2019 will see the release of "Apocalypse Now: The Final Cut," a 4K restoration that cuts roughly 20 minutes of footage that was added to "Redux."