Many years ago, I worked as a film and television extra for a few months. Interspersed with temporary jobs at insurance companies and accounting firms, the onscreen background work paid no more than short-term office stints, but provided welcome breaks of a day or two from hours of transcription and filing.   When the cameras start rolling, extras dine in cafes, walk down streets, or sit in airplane seats while the main characters live out their story arcs in the spotlight. Viewers scarcely notice the additional faces, but on-screen worlds would appear strangely empty—like the day after the a