Backed by the Pentagon: Or rather the Filipino military, who provided the F-5s for the napalm sequence and the helicopters for the famous helicopter attack preceding it. The helicopters occasionally had to be taken back for use in combat against Communist rebels.
Dawson Casting: Inverted. Laurence Fishburne lied about his age to get the role, as he was only 14 years old at the time. In an odd way, it makes the film better, showcasing such a young man in such a horrible place. It makes his death that much more of a tearjerker. By the end of the long production, he was 17, the same age as his character.
Enforced Method Acting: Martin Sheen also punched a mirror for real in his introductory scene where he has a psychotic break in his hotel room. So all that blood on the sheets? His. His idea, too. To shoot this scene, Coppola basically just gave Sheen as much whiskey as he could drink, put him in a room, and filmed the results. Apparently, Sheen's behavior was so disturbing to the camera crew that they wanted to stop the shoot, but Sheen insisted they press on. You can see the results for yourself. In Hearts of Darkness, the scene is shown making-of style. Coppola directs Sheen to shadowbox at the mirror, and Sheen (as noted, very drunk) misjudges his aim.
Also, this was the case (albeit unintentionally) with Dennis Hopper and Marlon Brando. According to Hopper, after Brando yelled at him over a simple misunderstanding he then decided to deliberately antagonize Brando whenever he could. This resulted in Brando refusing to share the set with him and the one scene they share together being shot on separate nights. So when Kurtz throws the book at Hopper's photojournalist character and calls him a "mutt" one can only assume that's Brando's genuine feelings about him.
Notice, too, the books read by Kurtz: 'From Ritual to Romance' and 'The Golden Bough'. Both are identified by T. S. Eliot's notes to 'The Waste Land' as key to the work, which was (prior to Ezra Pound's edit) to contain the epigraph 'The Horror! The Horror!'.
By the end of production, the film was over nine months behind schedule.
Coppola lost 100 pounds, threatened suicide several times, and attempted it once.
Coppola shot literally millions of feet of footage, and had to cut it all together to make a coherent motion picture.
Many crewmembers were drunk or stoned while filming. Dennis Hopper got 14-year-old Laurence Fishburne hooked on heroin.
Marlon Brando showed up to the set morbidly obese, rather than with the muscular build that his character called for (which is why Kurtz is almost never shown below the shoulders during the movie), and having not read either the script or Heart of Darkness like he had been asked.
Not that his having read the script would have mattered since it was constantly being re-written on the fly and Coppola had little to no idea how he wanted to end the movie.
Martin Sheen had a heart attack due to the stress of filming, and had to struggle for a quarter-mile to get help.
The budget was expected to be about $12-14 million, but wound up being well over double that amount ($31.5 million).
The helicopters that the government of the Philippines lent to Coppola for the famous "air cavalry" scene were frequently taken back during shooting, as the government needed the helicopters to fight communist rebels.
The original director assigned this script? George Lucas. God knows what kind of film this almost became. Cracked.com also makes one interesting assumption. Considering how long this movie dragged on, it's possible that if Lucas made this, he never would have gotten around to making Star Wars. According to Coppola's audio commentary, a documentary-style 16mm film shot in northern California with a couple of helicopters.
John Milius' early drafts had a less nihilistic ending, with Kurtz going out fighting against an overwhelming NVA attack, and Willard returning to America to take the news to Kurtz's wife and son. Willard's predecessors also played a larger role.
Harvey Keitel was cast as Willard first but was fired after two weeks. Al Pacino was considered but had the foresight to know how horrible the shoot would be.
Lynda Carter was cast as one of the Playboy Bunnies, but when the typhoon delayed production she had to go back to the States to continue with Wonder Woman and Colleen Camp was brought in to replace her. (There's a centerfold shot of Carter briefly visible in the movie and still floating around online, though some question its authenticity.)
Coppola first offered the role of Kurtz to Orson Welles (who had previously tried to adapt Heart of Darkness to the screen himself), but for some reason or another he declined. The documentary on the film includes the audio of a radio version that Welles did during his prime.
In the special features on the "Complete Dossier" edition, its said that Coppola wanted the film to be a special event by having it play in ONE theater somewhere in Kansas in the geographical center of the country built especially for the film with a specially made sound system where the film would run continuously for ten years and then hopefully anybody who wanted to show the film in their theaters would have to approach Coppola and exhibit it on his terms.
Kurtz's character was called Leighley in the original script. Worth noting that the reason for the change from Kurtz to Leighley was due to a conversation between Marlon Brando and Francis Ford Coppola when the latter was trying to describe the role to the actor. Brando, never having read Conrad's novella, said that an American Colonel would never have a name like "Kurtz" and would instead have something more English. After reading the novella, and understanding the reference, Brando demanded that his character's name be changed BACK to Kurtz after the film's completion, and Ford's lines were dubbed.
In the director's commentary Coppola spoke of how the ending for the movie was not figured out for a long time. Originally he had intended for Kurtz and Willard to fight off a massive NVA invasion of Kurtz' base and have Kurtz go out in apocalyptic intensity. The phrase, "The horror..." would still be uttered by Kurtz nonetheless. Eventually Coppola decided upon letting Willard assassinate Kurtz as the plot had been implying was going to happen.