Funnily enough, I just finished reading Graham Greene's novella last week. I didn't realise that he was asked to write the script/screenplay originally for the film, but he couldn't just write the script (he was a novelist, after all): he had to write the book first to work out the plot in his head. Then he changed parts of that to make it work for the film, especially for the 'Murican market. Most interestingly, he said that the film was superior to the book.
