Clint Eastwood wore the same poncho through all three "Man with No Name" movies without replacement or cleaning.
Orson Welles warned Sergio Leone not to make this movie on the grounds that Civil War pictures were box office poison.
Jack Elam turned down the role of Elam, the one-armed gunslinger who attempts to kill Tuco in the bathtub.
Eli Wallach claims that Sergio Leone decided that Tuco would carry his pistol on a lanyard and stuck in his belt rather than a holster because Wallach told him he always had trouble putting a pistol in a holster without looking at it.
Eli Wallach remembered that when he first came to Madrid all the hotels were full. Clint Eastwood invited him to sleep over at a friend's house and they shared the same bed. Wallach's wife Anne Jackson told him he could boast that he was the only man to sleep with Clint Eastwood.
Eli Wallach was almost poisoned on the set after drinking acid used to burn the bags filled with gold coin to make them rip open easier when struck with the spade. The acid had been poured into a lemon soda bottle and Wallach didn't know it. He drank a lot of milk and filmed the scene with a mouth full of sores.
Eli Wallach would have been decapitated during the train scene if he had lifted his head up. In the wide-shot, you can see the step that would have impacted his head.
Charles Bronson was offered both the roles of Tuco and Angel Eyes (the latter because Sergio Leone feared that audiences would not take kindly to Lee Van Cleef going from the fatherly, likable Col. Mortimer to a sneering villain. He declined both.
Mario Brega appears in all 3 of the Dollars Trilogy movies, and in all 3 movies, his character meets an unfortunate demise. In this movie, his character of Wallace is killed when Tuco (chained to him) jumps off the train with him, and bangs his head against some rocks. This is the only movie from the trilogy in which Brega plays an American, whereas in the other movies, he plays a Mexican.
Ennio Morricone's iconic theme music was designed in places to mimic the sound of crying hyena.
According to Eli Wallach, when it came time to blow up the bridge, Sergio Leone asked the Spanish army captain in charge to trigger the fuse, as a sign of gratitude for the army's collaboration. They agreed to blow up the bridge when Leone gave the signal "Vai!" (Go!) over the walkie-talkie. Unfortunately, another crew member spoke on the same channel, saying the words "vai, vai!", meaning "it's OK, proceed" to a second crew member. The captain heard this signal, thought it was for him and blew the bridge; unfortunately, no cameras were running at the time. Leone was so upset that he fired the crewman, who promptly fled from the set in his car. The captain was so sorry for what happened that he proposed to Leone that the army would rebuild the bridge to blow it up again, with one condition: that the fired crewman be re-hired. Leone agreed, the crewman was forgiven, the bridge was rebuilt and the scene was successfully shot.
According to Eli Wallach's autobiography "The Good, the Bad and Me", Sergio Leone picked him for the role of Tuco not because of his role as Calvera in The Magnificent Seven as most people assumed but rather because of his brief role as a Tuco-like bandit in How the West Was Won.
After Eli Wallach agreed with Sergio Leone that Tuco would carry his pistol on a lanyard, the director asked him to grasp the gun by shaking his neck, thus making the gun land in his hand. Wallach claimed that he wasn't able to do the intended action, and asked Leone to demonstrate it. When Leone tried, the pistol missed the director's hand and hit his crotch. Leone then told Wallach to hold the gun in the belt.
Although Clint Eastwood is usually top-billed in this film's credits, Eli Wallach has the most screen-time.
Because Sergio Leone spoke barely any English and Eli Wallach spoke barely any Italian, the two communicated in French.
Besides Clint Eastwood of course, actors Mario Brega, Benito Stefanelli, Aldo Sambrell, and Antonio Molino Rojo are the only actors to appear in all 3 of the "Dollars Trilogy" movies. Even though this movie does not have the word "Dollars" in its title, it is grouped with Per un pugno di dollari ("A Fistful of Dollars") and Per qualche dollaro in più ("For A Few Dollars More") as part of the "Dollars Trilogy".
Despite being frequently referred to as a sequel to Per qualche dollaro in più ("For a Few Dollars More"), this film is set during the American Civil War whereas that one takes place afterwards, and while Lee Van Cleef here plays a villain who gets killed, he turns up there as a very much alive good guy.
Due to the striking height difference between Clint Eastwood and Eli Wallach of over 9 inches, it was sometimes difficult to include them in the same frame.
During the scene right before the final duel where Tuco (Eli Wallach) is running frantically through the cemetery, a dog can be seen running on-screen at the beginning of the scene. In reality, that was improvised on the spot. Sergio Leone, who was afraid that the scene was going to slip into melodrama, released the dog without informing Eli Wallach first - thus, his look of surprise is quite genuine.