More than a year had passed between the filming of Willard and Chef searching the jungle for mangoes and encountering the tiger, and the immediately following shots (part of the same scene) of Chef clambering back onto the boat, ripping off his shirt and screaming.
The Philippines had no professional film laboratories at the time, meaning the raw camera negatives had to be shipped to the U.S. to be processed. Thus, the entire movie was shot blind. Francis Ford Coppola never saw a shot on film until after returning to California.
An early scene where Captain Willard is alone in his hotel room was completely unscripted. Martin Sheen told the camera crew to just let the cameras roll. Sheen was really drunk. He punched the mirror, which was real glass, cutting his thumb. Sheen also began sobbing and tried to attack director Francis Ford Coppola. The crew was so disturbed that they wanted to stop shooting, but Sheen wanted to keep the cameras going. At the time he was fighting a drinking problem and his own issues. He got so caught up in the scene and his own inner struggles that he hit the mirror. He believed that continuing the scene would help him face his problems. This was revealed later in a conversation with Coppola and Sheen, and has been shown in the Redux version.
Shooting, originally scheduled for six weeks, took 16 months.
The canteen scene with Lieutenant Colonel Kilgore and the wounded Viet Cong is based on an actual wounded VC fighter who fought while keeping his entrails strapped to his belly in an enameled cooking pot. The incident was documented by the photojournalist Philip Jones Griffiths. The real-life U.S. soldier was quoted as saying, "Any soldier who can fight for three days with his insides out can drink from my canteen any time!"