Here's Greene's preface to the book:
"The Third Man" was never written to be read but only to be seen. Like many love affairs, it started at a dinner table and continued with many headaches in many places, Vienna, Venice, Ravello, London, Santa Monica.
Most novelists, I suppose, carry round in their heads or in their notebooks the first ideas for stories that have never come to be written. Sometimes one turns them over after many years and thinks regretfully that they would have been good once, in a time now dead. So twenty years back, on the flap of an envelope, I had written an opening paragraph:
"I had paid my last farewell to Harry a week ago, when his coffin was lowered into the frozen February ground, so that it was with incredulity that I saw him pass by, without a sign of recognition, among the host of strangers in the Strand."
I, no more than my hero, had pursued Harry, so when Sir Alexander Korda asked me to write a film for Carol Reed to follow our "Fallen Idol," I had nothing more to offer than this paragraph. Though Korda wanted a film about the four-power occupation of Vienna, he was prepared to let me pursue the tracks of Harry there.
To me it is almost impossible to write a film play without first writing a story. Even a film depends on more than plot, on a certain measure of characterization, on mood and atmosphere, and these it seems to me almost impossible to capture for the first time in the dull shorthand of a script. One can reproduce an effect caught in another medium but one cannot make the first act of creation in script form. One must have the sense of more material than one need to draw on. "The Third Man," therefore, though never intended for publication, had to start as a story before it began those apparently interminable transformations from one treatment to another.
On these treatments Carol Reed and I worked closely together, covering so many feet of carpet a day, acting scenes at each other. No third ever joined our conferences: so much value lies in the clear cut-and-thrust of argument between two people. To the novelist, of course, his novel is the best he can do with a particular subject; he cannot help resenting many of the changes necessary for turning it into a film or a play. But "The Third Man" was never intended to be more than the raw material for a picture. The reader will notice many differences between the story and the film, and he should not imagine these changes were forced on an unwilling author: as likely as not they were suggested by the author. The film, in fact, is better than the story because it is in this case the finished state of the story.
Some of these changes have obvious superficial reasons. The choice of an American instead of an English star involved a number of alterations. For example, Mr. Joseph Cotten quite reasonably objects to the name Rollo. The name had to be an absurd one, and the name Holley occurred to me when I remembered that figure of literary film, the American poet Thomas Holley Chivers. An American, too, could hardly have been mistaken for the great English writer Dexter, whose literary character bore certain echoes of the gentle genius of Mr. E.M. Forster, so that the confusion of identities would have been impossible, even if Carol Reed had not rightly objected to a rather far-fetched situation involving a great deal of explanation that increased the length of a film already far too long.
Another minor point: in deference to American opinion, a Rumanian was substituted for Cooler, since Mr. Orson Welles' engagement had already supplied us with one American villain. (Incidentally, the popular line of dialogue concerning Swiss cuckoo clocks was written into the script by Mr. Welles himself.)
One of the very few major disputes between Carol Reed and myself concerned the ending, and he has been proved triumphantly right. I held the view that an entertainment of this kind, which in England we call a thriller, was too light an affair to carry the weight of an unhappy ending. Reed on his side felt that my ending — indeterminate though it was with no words spoken — would strike the audience, who had just seen Harry die, as unpleasantly cynical. I admit I was only half-convinced: I was afraid few people would wait in their seats during the girl's long walk from the graveside and that they would leave the cinema under the impression of an ending as conventional as mine and more drawn-out. I had not given enough consideration to the mastery of Reed's direction, and, at that state, of course, we neither of us could have anticipated Reed's brilliant discovery of Mr. Karas, the zither player.
The episode of the Russians kidnapping Anna (a perfectly possible incident in Vienna) was eliminated at a fairly late stage. It was not satisfactorily tied into the story, and it threatened to turn the film into a propagandist picture. We had no desire to move people's political emotions: we wanted to entertain them, to frighten them a little, to make them laugh.
Reality, in fact, was only a background to a fairy tale; none the less, the story of the penicillin racket is based on a truth all the more grim because so many of the agents were more innocent than Joseph Harbin. The other day in London a surgeon took two friends to see the film. He was surprised to find them subdued and depressed by a picture he had himself enjoyed. They then told him that at the end of the war when they were with the Royal Air Force they had themselves sold penicillin in Vienna. The possible consequences of their act had never before occurred to them.
So who knew that for two years in the early Fifties it was also a radio show?
You can download episodes over at Old Time Radio Fan.