*The Third Man* without Orson Welles as Harry Lime? It almost happened:
"What Welles didn't know was that his getting the part of Harry Lime at all--the part that was to be the most popular he ever played, in the most successful film in which he ever participated--had been very touch-and-go. Reed and Korda were keen (Korda in part because he wanted to redeem at least some of their three-picture deal contract), but Korda's American partner on the film, David O. Selznick, was bitterly opposed to Welles' involvement from the beginning. Cary Grant, Noel Coward and David Niven had already been proposed as alternatives; finally, when the two central male characters were reconceived as Americans, Robert Mitchum was keenly championed as Lime by Selznick. Welles was saved by the bell on that one when Mitchum, one of America's top grossers, was arrested for possession of marijuana, though Selznick continued to insist that casting Welles would be 'a detriment'to the picture: he was box-office poison--specially commissioned Gallup polls had proved it. 'While I do not profess knowing as much as Mr Gallup about box office values,' replied Korda in the feline tone he deployed when dealing with his pesky American running mate,'I cannot believe him being a detriment. Carol thinks Orson could give a tremendous performance in this part. Picture greatly depends on Lime being extraordinary in attraction and superior in intellect."
"Selznick seemed to accede, at which point Welles's *Macbeth* opened in America to villainously bad reviews, causing Selznick to exclaim that Welles would be a far more damaging name 'than has been in our worst fears to date'. How about Rex Harrison for the part? Korda drily informed Selznick that it was too late: Welles had been signed. Which, as it happened, he hadn't, though he would be soon enough. Korda was satisfied; as early as 1947 he had written a memo in which he stated his intention of making 'a photoplay, as yet untitled, to be directed by Carol Reed, starring Orson Welles'. The irony of these casting shenanigans is that the description of Harry in the novella Graham Greene wrote, before embarking on the screenplay and long before any casting was contemplated, is an almost precise description of Welles: 'Don't picture Harry Lime as a smooth scoundrel,' says Major Calloway. 'The picture I have of him on my files is an excellent one: he is caught by a street photographer with his stocky legs apart, big shoulders a little hunched, a belly that has known too much good food for too long, on his face a look of cheerful rascaliry, a geniality, a recognition that his happiness will make the world's day." On the other hand, Selznicks doubts about Welles's box-office appeal were not unfounded--that is, until *The Third Man*, which made him an international star...."
https://books.google.com/books?id=LyEoCgAAQBAJ&pg=PT30
Of course with anyone else it becomes a very different movie, not least because it will lack Welles' most famous lines:
"You know what the fellow said--in Italy, for thirty years under the Borgias, they had warfare, terror, murder and bloodshed, but they produced Michelangelo, Leonardo da Vinci and the Renaissance. In Switzerland, they had brotherly love, they had five hundred years of democracy and peace--and what did that produce? The cuckoo clock."
These lines were not in Greene's script. "Greene always openly acknowledged Welles' authorship of these famous lines. Welles in turn graciously insisted that he had cribbed them from a play by a Hungarian whose name he had forgotten, which he had seen when he was a little boy visiting Vienna with his father."
https://books.google.com/books?id=LyEoCgAAQBAJ&pg=PT37
(It would be pedantic to point out that the cuckoo clock was probably invented in southwest Germany, not in Switzerland...)