Although inspired by Joseph Conrad's Heart of Darkness, the film deviates extensively from its source material. The novella, based on Conrad's experience as a steamboat captain in Africa, is set in the Congo Free State during the 19th century. Kurtz and Marlow (who is named Willard in the movie) work for a Belgian trading company that brutally exploits its native African workers.
When Marlow arrives at Kurtz's outpost, he discovers that Kurtz has gone insane and is lording over a small tribe as a god. The novella ends with Kurtz dying on the trip back and the narrator musing about the darkness of the human psyche: "the heart of an immense darkness".
In the novella, Marlow is the pilot of a river boat sent to collect ivory from Kurtz's outpost, only gradually becoming infatuated with Kurtz. In fact, when he discovers Kurtz in terrible health, Marlow makes an effort to bring him home safely. In the movie, Willard is an assassin dispatched to kill Kurtz. Nevertheless, the depiction of Kurtz as a god-like leader of a tribe of natives and his malarial fever, Kurtz's written exclamation "Exterminate the brutes!" (which appears in the film as "Drop the bomb. Exterminate them All!") and his last words "The horror! The horror!" are taken from Conrad's novella.
Coppola argues that many episodes in the film—the spear and arrow attack on the boat, for example—respect the spirit of the novella and in particular its critique of the concepts of civilization and progress. Other episodes adapted by Coppola, the Playboy Playmates' (Sirens) exit, the lost souls, "take me home" attempting to reach the boat and Kurtz's tribe of (white-faced) natives parting the canoes (gates of Hell) for Willard, (with Chef and Lance) to enter the camp are likened to Virgil and "The Inferno" (Divine Comedy) by Dante. While Coppola replaced European colonialism with American interventionism, the message of Conrad's book is still clear.
Coppola's interpretation of the Kurtz character is often speculated to have been modeled after Tony Poe, a highly decorated Vietnam-era paramilitary officer from the CIA's Special Activities Division. Poe's actions in Vietnam and in the 'Secret War' in neighbouring Laos, in particular his highly unorthodox and often savage methods of waging war, show many similarities to those of the fictional Kurtz; for example, Poe was known to drop severed heads into enemy-controlled villages as a form of psychological warfare and use human ears to record the number of enemies his indigenous troops had killed. He would send these ears back to his superiors as proof of the efficacy of his operations deep inside Laos. Coppola denies that Poe was a primary influence and says the character was loosely based on Special Forces Colonel Robert B. Rheault, whose 1969 arrest over the murder of suspected double agent Thai Khac Chuyen in Nha Trang generated substantial contemporary news coverage.
Use of T. S. Eliot's poetry
In the film, shortly before Colonel Kurtz dies, he recites part of T. S. Eliot's poem "The Hollow Men". The poem is preceded in printed editions by the epigraph "Mistah Kurtz – he dead", a quotation from Conrad's Heart of Darkness.
Two books seen opened on Kurtz's desk in the film are From Ritual to Romance by Jessie Weston and The Golden Bough by Sir James Frazer, the two books that Eliot cited as the chief sources and inspiration for his poem "The Waste Land". Eliot's original epigraph for "The Waste Land" was this passage from Heart of Darkness, which ends with Kurtz's final words:
Did he live his life again in every detail of desire, temptation, and surrender during that supreme moment of complete knowledge? He cried in a whisper at some image, at some vision, – he cried out twice, a cry that was no more than a breath –"The horror! The horror!"
When Willard is first introduced to Dennis Hopper's character, the photojournalist describes his own worth in relation to that of Kurtz with: "I should have been a pair of ragged claws/Scuttling across the floors of silent seas", from "The Love Song of J. Alfred Prufrock".