Comedian Redd Foxx received special mention in the closing credits, for the use of one of his jokes in this exchange between Bill (Richard Gere) and Linda (Linda Manz): "I saved your life today." "How?" "I killed a shit-eating dog."
Shot almost entirely at "magic hour," the hours between day and night early in the morning and late in the evening. Terrence Malick wanted to have a white sky and no sight of the sun. This was the first film to utilize a new Eastman ultra light-sensitive stock negative which enabled clarified images to be shot in the magic hour; at dawn, at dusk and into the night.
Cinematographer Néstor Almendros was going blind during production. Before each shot, he would have his assistant take a picture with a Polaroid camera and then would view it under a high-powered magnifying glass.
In an interview conducted for the Criterion Collection DVD edition of the film in 2007, Richard Gere speaks of a shot in the film during the wheat fire where a "monstrous" antiquated tractor is driven through the flames. According to Gere, director Terrence Malick was the driver.
The film's title is a reference to Deuteronomy 11:21 - "That your days may be multiplied, and the days of your children, in which the LORD swore to your fathers to give them as the days of heaven upon the earth."
The screenplay is loosely based on the Milady back story featured in "The Three Musketeers" by Alexandre Dumas. In the Dumas novel, Milady was originally born Anne de Bueil and became a thief with her lover, a priest who had renounced his orders. On the run, they pretend to be brother and sister and hide in a village. She ends up seducing and marrying for her own interests the local nobleman. In most of the adaptations of the novel, the back story is missing, but the Italian adaption Milady and the Musketeers (1952) focuses on the very same events depicted in 'Days of Heaven (1978)', naturally with very different character motivations.
Despite the film's commercial failure, Charlie Bluhdorn, who ran Paramount's parent company Gulf+Western, loved it so much that he offered Terrence Malick $1 million for his next project, whatever it was.
The visual motif of the far-off farmhouse surrounded by wheat fields is reminiscent of Andrew Wyeth's 1948 painting "Christina's World" as well as Edward Hopper's painting "House by the Railroad". It is also reminiscent of Reata, the ranch home of the Benedict Family in Giant (1956).
After finishing the film, Terrence Malick began to develop a project for Paramount entitled "Q". Riddled with production troubles, Malick would abandon this project after a brief span of second unit location shooting. He would not make another film until The Thin Red Line (1998), twenty years later. Ideas that began with the abandoned "Q" movie would later be re-worked into The Tree of Life (2011) and Voyage of Time: Life's Journey (2016).
When Lynda sits in the farmer's house reading a book that has illustrations of animals, including a tiger, water buffalo, and a snake, the book is an illustrated edition of Rudyard Kipling's "The Jungle Books".
The film features a brief scene of dogs hunting the prairie. The German Shorthaired Pointer (GSP) shown is Jocko von Stolzhafen, twice GSP National Champion (Field) and perhaps the best GSP of his era. A year or so later Jocko vanished while running at a training camp, presumably stolen.
According to 'Carrie Fisher', she was interviewed for a role and even read with 'John Travolta'; when he couldn't do the film and Fisher read opposite 'Richard Gere', the chemistry she had with Travolta wasn't there, which she suspects is why she wasn't cast.