David Letterman, Bill Murray to mark 20 years of 'The Late Show' on CBS

On Thursday night, David Letterman marks 20 years on CBS, a length of time that is nearly one-third the age of the network's average viewer (cue rimshot). His guest will be fellow sexagenarian Bill Murray, who was also the guest on Letterman's first show on CBS, in August 1993, and his first late-night show ever, on NBC in 1982. If the pair's 24 previous interviews on "The Late Show" are any indication, this one will involve a costume, a song, some kind of gag, and the trading of unpleasantries ...