Hey I saw this (animated/live)docupic about this underground cartoonist, who was a guest on Letterman's a bunch of times but eventually walked off in an angry rage? That came back in the film too ... wait, lemme look up what I'm talking about ...yeah, film was called American Splendor, guy is Harvey Pekar. Anyone know what I'm talking about?
Harvey Pekar really nailed him, and rightly so. The reason they had an actor playing Letterman is because they wouldn't allow the director to use the original footage. Pekar made Letterman look bad.
I once saw Charles Grodin make Letterman look really bad too. Grodin refused to go along with the routine.
There's another interesting story about this guy named "Pete" the dishwasher. He's a real life guy whose goal was to wash dishes in every state while he wrote a darkly comic journal on it which he published.
Pete told the story of how he was contacted by the show to be a guest-- he wasn't interested in going on the air, but he was interested in the money, so a friend went on the air for him instead. Pete went along with him to New York and enjoyed all the perks of being on the show, the food, the hotel room, and the money. The show figured it out that the guy who went on as a guest wasn't the real Pete. The only reason they didn't sue was because it wouldn't be worth their time or the cost to take Pete to court.