The British writing-directing team of Michael Powell and Emeric Pressburger were shown a pre-release screening of the film by producer David O. Selznick. Both were thoroughly unimpressed with the movie, but didn't want to offend Selznick by saying so. At the end of the film, when Gregory Peck and Jennifer Jones are crawling towards each other on a mountain and when they get near each other they both open fire, Pressburger turned to Powell and whispered, "What a pity they didn't shoot the screenwriter".
This film's musical score was the subject of a famous soundstage exchange between producer David O. Selznick and composer Dimitri Tiomkin. When Selznick first heard Tiomkin's "love theme", he was visibly disappointed and admonished the composer, "You don't understand. I want real f**king music!" To which Tiomkin angrily replied, "You f**k your way, I f**k my way. F**k you - I quit!" Their differences were eventually patched up, and Tiomkin's music was used in the final film.
David O. Selznick had originally intended this property as his artistic follow-up to Gone with the Wind (1939). He envisioned a lavish production with no expense spared, and ultimately he got his wish. Constant production delays, many caused by Selznick's meddling and the hiring and firing of as many as seven directors (including Selznick himself), as well as an extended editing period to cut the film from its original 26-hour running time, caused the budget to balloon to a then-horrifying sum of $6 million, plus an additional $2 million in marketing costs. Though the film eventually did turn a profit, it effectively marked the end of Selznick's career. However, he went on to produce prestige films such as The Paradine Case (1947), Portrait of Jennie (1948), The Third Man (1949) and A Farewell to Arms (1957).
The "purity" medal that Walter Huston gives to Jennifer Jones is an Egyptian Magic Coin token, a form of the "Good Luck" token. These were produced starting around 1905, as one appears in a Sears & Roebuck catalog then. Many more were produced in the 1920s, after the discovery of King Tut's tomb made news all over the world. One side has an Egyptian pharaoh and the other side has an Egyptian sphinx and pyramids. They are usually around 32mm in diameter and made of brass. Some are plated with silver or white metal.
Producer (and uncredited director) David O. Selznick battled amphetamine addiction throughout production. His drug abuse exacerbated much of his erratic behavior during filming, including his constant demand for reshoots.
Gregory Peck's work on The Yearling (1946) overlapped for three or four weeks with this film. Peck would work on "The Yearling" in the morning and "Duel in the Sun" later in the day. Accordimg to the actor, "I didn't do much acting. I rode horses, necked with [Jennifer Jones] and shot poor old [Charles Bickford].
Dice, Gregory Peck's horse in the film, made "Life Magazine" because, while the movie was being filmed, Peck rode him into the dining room of the Hotel Santa Rita, then through the hotel lobby, entered an open elevator, decided it was too small, backed out, and climbed the stairs as well.