Before writing the screenplay, Greene worked out the atmosphere, characterization, and mood of the story by writing a novella. This was written purely to be used as a source text for the screenplay and was never intended to be read by the general public, although it was later published
The atmospheric use of black and white expressionist cinematography (by Robert Krasker), with harsh lighting and distorted camera angles, is a key feature of The Third Man. Combined with the unique theme music, seedy locations, and acclaimed performances from the cast, the style evokes the atmosphere of an exhausted, cynical post-war Vienna at the start of the Cold War.
I found this great diagram in the book 'On Film Making' by Alexander Mackendrick. Mackendrick was the writer and director of 'The Man in the White Suit' and 'The Sweet Smell of Success' as well as the director of 'The Lady Killers' (among others). This book is an absolute gem, brimming with great advice.
Through the diagram you can see how intricately Graham Greene had worked out all of his complex relationships. You also see how contradictions stand to create conflict and a help create a richness of character. In feature films it is difficult to create characters with any degree of depth and that is why Greene wrote his first draft as a novella and then transplanted it into a screenplay.