I'm fairly sure it was Cagney in the same movie (Cagney could play crazy like nobody else!). I can't recall the exact circumstances (it was 57 years ago that I saw the movie!), but they're about to smuggle some hood in the trunk of a car (I think he may have ratted on the Cagney character, but thought he was getting away with it). He crawls into the trunk and they close the lid. His muffled voice comes out of the trunk and he's complaining that there's no air in there. Cagney says, "Here! Lemme give ya some air holes!" Then he pulls his gun out of his shoulder holster and empties it into the trunk lid.

You could hear gasps of horror all over the theater.

Don Firth