Fake Nationality: Eli Wallach, a Polish-American Jew, plays the Mexican Tuco. Wallach also portrayed a Mexican in his second most memorable role, as the bandit Calvera in The Magnificent Seven. Also Spanish Antonio Casas and Italians Livio Lorenzon and Antonio Casale as Americans Stevens, Baker and Jackson, and Italians Aldo Giuffre and Mario Brega as Americans Captain Clinton and Corporal Wallace.
Fatal Method Acting: Almost happened a few times during the production. In the scene where the bridge is detonated, you can see a massive chunk of debris flying past Clint Eastwood and just barely missing his head.
Eli Wallach cheated death at least five times making the film. When Wallach had to lie by train tracks as a train went by he wasn't warned that if he raised his head too high the steps jutting from the cars (which weren't accounted for when figuring out safety concerns) would have taken his head off. Fortunately he had insisted that the first take be kept and it was. On another occasion a crew member put a bottle of acid right beside his drink and he was almost poisoned when he drank from it (he had the sense to spit it out right away). And yet another time when he had his hands bound and was sitting on a horse during the scene where Blondie shoots the rope, the horse got too spooked and ran a mile before anyone could stop it, with Wallach stuck on top. Not to mention how incredibly lucky he was to have been alive after that one time the pyrotechnics didn't cut the rope properly.
Just Train Wrong: The military train scene. The engine is quite clearly a Spanish engine with a cowcatcher and balloon smokestack clapped on, and the buffers are clearly visible. As the train moves on, we can see the European-style two-axle cars, instead of the American bogie cars, which were well established by the 1860s.
Mean Character, Nice Actor: All three of the stars. On camera, Blondie is cold, distant, intimidating and trusts no one; Angel Eyes is just a complete remorseless psycho; and Tuco is a scheming rascal who would only sweet-talk anyone if it serves his benefit. Off camera, Clint is a generous person who helped Eli to survive the perils of a Spaghetti Western, Lee was an utter gentleman who refused to hit Rada Rassimov properly when she asked for it, and Eli was a genuinely warm, personable and friendly man who hated guns and had no use for them.
Na´ve Newcomer: Eli was new to the Spaghetti Western genre, and wasn't quite prepared for the world of pain he was in for.
Clint Eastwood apparently warned Wallach not to trust the Italian film crew because of how dangerous it could become during filming. See Fatal Method Acting above for the ways in which poor Eli suffered. Also, the bridge went off at the wrong time.
Speaking of the bridge scene, if you look closely, a huge piece of debris lands about two feet from Clint's head as he's behind the sandbags. And you all thought it was for effect.
The Other Darrin: Some scenes from the Italian original were cut from the first English-language version and didn't get put back in until 2003. Clint Eastwood and Eli Wallach were still available for the English dub of these scenes, but since Lee Van Cleef was dead, Simon Prescott did his voice.
Tuco's line "When you have to shoot, shoot, don't talk!" was actually improvised by Wallach, which apparently caused the whole crew to burst out laughing. Eli was a little perplexed because he thought that what he said was actually pretty sensible; that is, he didn't mean it as a joke, but his delivery and the look on his face made it side-splittingly hilarious.
Tuco playing with the guns in the gun shop was improvised with permission from Sergio Leone, as Eli Wallach had no idea how guns really worked. The exasperated look in the seemingly feeble shop owner's face is real. Couples with Harpo Does Something Funny
Also, Tuco shoving the "Open/Closed" sign in the gun shop owner's mouth was Eli's idea.
What Could Have Been: Entire sequences were cut from the film for time. For example, when Tuco would have been blown off his horse by cannonfire, he'd have tried to fire back at Blondie with a nearby cannon, but of course the clumsy bandit keeps misfiring and the cannon starts falling apart.