According to "Hearts of Darkness: A Filmmaker's Apocalypse" what went wrong during the production of the film?

Answer 1

a lot of stuff.
-Harvey Keitel was then cast as Willard. Two weeks into shooting, director Francis Ford Coppola replaced him with Martin Sheen.
-Francis Ford Coppola believed that Marlon Brando was familiar with Joseph Conrad's "Heart of Darkness" and had prepared for the role before the legendary actor arrived on the set. When Brando did come out, Coppola was horrified to find that Brando had never read "Heart of Darkness", did not know his lines, and had become extremely fat (Kurtz had always been written as a tall but starvingly-thin man). After some panicking, Coppola decided to film the 5'10" Brando as if he was a massively built, 6'5" brute (to explain Brando's size) and steered the camera clear of Brando's huge belly.
-Marlon Brando so angered Francis Ford Coppola that the director turned over the filming of Brando's scenes to Jerry Ziesmer, the assistant director.
-Martin Sheen was actually drunk in the scene where Willard is drunk alone in his hotel room. All of Sheen's actions in that scene were a result of his real intoxication. When Sheen punched the mirror (which was real glass), he really did cut his hand as shown in the film. While drunk, Sheen also began sobbing and tried to attack Francis Ford Coppola.
-Martin Sheen had a heart attack during the filming and some shots of Willard's back are of doubles, including Sheen's brother who was flown out specially. Coppola was so worried that backing would be withdrawn by the studio and distributor if news of Sheen's heart attack leaked out, that he kept it quiet, even to the extent of explaining Sheen's hospitalization as being due to "heat exhaustion" in the official Shoot Schedule.
-Coppola shot nearly 200 hours of footage for this film.
-A typhoon destroyed sets, causing a delay of several months.
-Originally scheduled to be shot over six weeks, ended up taking 16 months.

Plus much more... look at the link in my source list to find out more.

Answer 2

The star, Martin Sheen had a heart attack during filming.

Answer 3

Marlon Brando was overweight for the part, and never read the script before showing up.