This isn't, strictly speaking, the first sentence of Graham Greene's screenplay for The Third Man, nor is it the first sentence of the novel he later wrote from the screenplay. But it is the first sentence that got the whole ball rolling. From the author's Preface to the novel:
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 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.'Some time later Greene was approached to write a screenplay to follow up on The Fallen Idol, and he returned to Harry. But the producer wanted a story about the four-power occupation of Vienna after WWII, so it wasn't long before the only thing left of Harry was his first name, his unexpected reappearance, and the frozen February ground into which he was supposed to have been lowered.
Greene shared this interesting account of his own inventional process, and why the materials for the novel were hanging around after the screenplay was produced:
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 seem to me to be 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 needs. The Third Man, therefore, though never intended for publication, had to start as a story before those apparently interminable transformations from one treatment to another.