There are certain guests Late Show host David Letterman just has to have on, by dint of popularity or importance, and you got the feeling he was getting the worst out of the way all at once last night. Letterman's lineup started with humor-challenged Fox News shouting head Bill O'Reilly before moving on to empty-vessel Hills star Lauren Conrad. There were plenty of painful moments. But then there were also delightful interludes in which Letterman couldn't help but slice into his guests. Click the video icon to watch the attached clip, in which Letterman basically calls Conrad an idiot before backing off in a fit of giggles and self deprecation.