Big-Lipped Alligator Moment: The random guy getting killed by some offscreen shooter right before the opening credits. This scene is never explained nor mentioned again. However, the scene is almost immediately followed by a narration describing the start of the bounty hunter profession, so one can assume it is one of the wanted criminals being ambushed and killed.
"Common Knowledge": Christopher Frayling's book Something To Do With Death claims that Gian Maria Volonte was "contractually obligated to dub his own dialogue into English," which has become a widely-repeated piece of trivia. In fact, Volonte is dubbed by American voice actor Bernard Grant, the same actor who dubbed him in A Fistful of Dollars; Volonte spoke very little English and likely couldn't have done so himself, "contractual obligation" or not.
Complete Monster: El Indio is a Mexican Bandito who obtains a secret from his cellmate, and then kills the latter so that he can keep the information for himself. He forces a former member of his gang to listen to his wife and infant child being slaughtered, then forces him into a duel, which he rigs in his own favor. When he discovers that Colonel Mortimer and Monco are bounty hunters, he has them tortured, then sets them free, killing one of his own men and framing them for it; his plan is to have his gang and the bounty hunters kill each other, leaving all the loot for him. When Nino, the one gang member he'd planned to keep around is killed in front of him, El Indio displays absolutely no reaction, and he's ultimately revealed to have killed Mortimer's brother-in-law and raped his sister, driving her to suicide.
Ho Yay: Mortimer and Manco's relationship eventually evolves into a gentle bromance.
Magnificent Bastard: Colonel Douglas Mortimer is a Bounty Hunter seeking revenge on gang leader El Indio for raping his sister. Introduced luring an outlaw out to kill him, Mortimer gets wind of Indios escape from prison and later encounters several of his men, nearly provoking one into a fight. Deciding to team up with another bounty hunter known as Manco (The Man with No Name), Mortimer sends Manco to infiltrate Indios gang and through him lures the gang to his location after they rob a bank, then steals the loot without Indio realizing hed opened it. After killing several of Indios men in a gun battle, Mortimer finds himself helpless at Indios hands and is resigned to his fate until Manco arrives to even the odds, whereupon Mortimer kills Indio in a Duel to the Death and rides into the sunset, his vengeance fulfilled.
Moral Event Horizon: El Indio seems to truly cross this when he has an innocent child and its mother shot to death offscreen to get back at the man who took money to put him behind bars. However, it is later revealed that he had crossed it long before the film began.
In the wonderful shooting contest scene, Monco walks around Mortimer, glaring at him; Mortimer walks around Monco, glaring at him; Monco steps on Mortimer's shoe, getting it muddy; Mortimer does the same to him, et cetera — until Mortimer proves himself to be the better shot by perforating Monco's hat, but up until then it's hilarious. Two little boys hang a lampshade on it: "Just like the games we know!"
Another one from Mortimer, when he spots a wanted poster of his nemesis. And looks at it very hard.
Monco and Mortimer calling each other "boy" and "old man" can seem this way if you know Lee Van Cleef was only five years older than Clint Eastwood.