Right at the beginning, when Holly Martins (Joseph Cotten) first comes to Harry Lime's place, a neighbour (or was it the caretaker?) informs him that Harry is dead, and says he doesn't know whether he went to heaven or hell. While saying it, he points upwards to heaven and downwards to hell. However, the shot is composed so that Holly is looking up a flight of stairs to this guy, and the guy is seen upside down, which means that the directions he points to are the exact reverse of what he intends. It's a simple device, but it does much to establish the world view that most characters in the movie have.
Although Orson Welles features prominently in the credits, his Harry Lime is almost a MacGuffin - much of the movie has to do with an investigation of the circumstances of his death, and the people connected to him. "We should've dug deeper than a grave," the British officer says at one point. By the third act, one has almost forgotten that he's listed in the credits when he appears suddenly, framed against a doorway, smiling that sardonic smile like only Orson Welles can. The impact of that shot is fantastic - again, an off-kilter composition, making him appear, both literally and figuratively, at an odd angle to the proceedings.
For most people, the ferris wheel scene with Orson Welles and Joseph Cotten is the high point. The tension in this scene, the dialogue that crackles with sardonic wit, the way it trusts the viewer to assemble the jigsaw without having to spell out what has really transpired until then... fantastic doesn't even begin to describe it.
Everyone, of course, remembers the cuckoo clock speech. Legend has it that this speech was of Welles' own devising - it is not in the Graham Greene novel, not did Greene write it in the script. I reproduce it here, simply for the sheer pleasure of quoting it:
Don't be so gloomy. After all it's not that awful. Like the fella says, in Italy for 30 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 500 years of democracy and peace, and what did that produce? The cuckoo clock.
The last scene is yet another of my favourites. After Harry Lime is well and truly buried, Holly hitches a ride with the officer to the railway station. Just then, he spies Anna (Alida Valli) walking down that road in his direction. He still has feelings for her, so he gets off and waits for her. She walks towards him, then past him, and slowly away. The book ends differently, on a happier note (for Holly at least). However, this ending is definitely more appropriate, given all that has happened. When I first saw this movie, I remember praying fervently for her to just keep walking and not go to him. I kept muttering "walkawaywalkawaywalkaway..." almost continuously. Thank goodness someone was listening!
Aside: Incidentally, one other movie where I muttered a similar prayer was Roman Holiday. As Gregory Peck walks away after meeting Audrey Hepburn for the last time, I kept praying that she shouldn't run after him, as she almost certainly would have in a lesser movie. A big reason why I love that movie is that he just keeps walking.