While the film is far more serious than the Dollars Trilogy, constant sarcasm from Harmonica and Frank occasionally crosses the line into hilarious.
Frank: How can you trust a man who wears both a belt and suspenders? The man can't even trust his own pants.
Guess where Frank shoots the informer when the idiot proves to be too much of a liability? Hint: Frank shoots him three times...
And Cheyenne's reaction to the informer's death is deadpan: "Told you to keep your mouth shut."
Also an in-joke. Leone himself wore a belt and suspenders.
Some scenes in the 11 minute intro, probably an intentional example. And the tense and cool exchange afterwards.
Harmonica: Did you bring a horse for me? Frank's Man: Looks like we're shy one horse. Harmonica: (shakes head) You brought two too many.
Harmonica: I saw three of these dusters a short time ago, they were waiting for a train. Inside the dusters, there were three men. Cheyenne: So? Harmonica: Inside the men, there were three bullets.
This little gem:
Harmonica: The reward for this man is 5000 dollars, is that right? Cheyenne: Judas was satisfied with 4,970 dollars less. Harmonica: There were no dollars in them days. Cheyenne: But sons-of-bitches? Yeah.