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In the theatrical trailer, Angel Eyes is "The Ugly" and Tuco "The Bad", which is the reverse of their designations in the actual movie. This is because the Italian title translated into English is actually "The Good, the Ugly, the Bad", not "The Good, the Bad and the Ugly", and the Italian trailer had "The Ugly" and "The Bad" in that order. When the trailer was transferred to English, "The Ugly" and "The Bad" were not reversed to coincide with the altered title, causing the incorrect designations.
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 okay, proceed" to a second crew member. The Captain heard this signal, thought it was for him, and blew up 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.
Because writer and director Sergio Leone spoke barely any English and Eli Wallach (Tuco) spoke barely any Italian, the two communicated in French.
Clint Eastwood wore the same poncho through all three "Man with No Name" movies without replacement or cleaning.
In the gun store, everything Tuco ( Eli Wallach ) does with the guns was unscripted. Wallach knew little about guns, so he was instructed to do whatever he wanted. He did receive instruction, however, on how to disassemble and assemble them. Most of the Storekeeper's (Enzo Petito's) bemused reactions throughout the scene are genuine. The scene where Tuco shoves the open or closed sign in his mouth was also Wallach's idea.
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. He didn't know the acid had been poured into a lemon soda bottle. He drank a lot of milk, and filmed the scene with a mouth full of sores.