If, as Howard Hawks insisted, the definition of a great movie is “three great scenes, no bad scenes,” then The Third Man qualifies easily. The reveal of Harry Lime in the doorway (the most famous entrance in the history of movies, as Roger Ebert said), the “cuckoo clock” speech, and that awesome, and thoroughly unsentimental, finale — all as great as anything I’ve ever seen in the movies.
Of course, the rest of it is pretty great, too.
By the way, the fact that Greene felt he had to write the story as prose before he wrote the screenplay is presented as some sort of absolute, at least in his view, but when Paul Thomas Anderson started out to adapt Inherent Vice the first thing he did was to write out the entire novel as a screenplay, because that’s the way he could start editing it.
The tools we’re used to using always feel most comfortable in our hands.