Everybody wants something...
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.
I was talking with a fellow screenwriter the other day and she was mentioning how novices were lacking a counter theme in their stories. That is to say that their antagonists and secondary characters are lacking a clearly defined goal. If you were to suddenly change the point of view to any of the characters you would see that the film would still work. If you made Anna's story in 'The Third Man' the central story (made her the protagonist) the story still works. It may not be the strongest point of view but in the end it is still functioning.