When the butler tells of Harry Lime's death, at the start of the conversation he can hardly speak English. As the conversation goes on, his English gets better and better, until at the end it's perfect with just an accent.
Orson Welles' line "...you know what the fellow said... In Italy for thirty years under the Borgias they had warfare, terror, murder, bloodshed - but they produced Michelangelo, Leonardo da Vinci, and the Renaissance. In Switzerland they had brotherly love, five hundred years of democracy and peace, and what did that produce? The cuckoo clock." was not in the script. Welles just wanted to say it, so he did, much to the anger of the original author. He thought it was a terrible piece of writing but it has turned out to be the most memorable line in the entire film.