Regis Philbin has appeared as a Late Show guest more than any other celebrity guest. He's tight with the hilarious guest.
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Julia Roberts has always been very close to Letterman. And she got especially close to him during this appearance.
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After being repeatedly asked to appear on the talk show, Cher didn't make it until 1987. Once on stage, she notoriously referred the host as an "a--hole."
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Letterman dedicated an entire 2002 episode to his close friend, musician Zevon, after the latter was diagnosed with terminal cancer. He was one of the only people ever to be the sole guest on an episode of The Late Show.
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Madonna has visited Letterman numerous times over the years, but her longest-lasting legacy stems from the 1994 appearance when she dropped the F-bomb 14 times.
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Joaquin Phoenix stopped by in 2009. He wore a giant beard and sunglasses and remained mostly quiet and sullen, allowing Letterman to have tons of fun at his expense.
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In 2006, Letterman was caught off guard when Sacha Baron Cohen appeared in character as Borat. The subsequent interview was awkward... and awesome.
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In 2007, Paris Hilton appeared on The Late Show... and Letterman peppered the former sex tape star about her stint in jail. He later apologized for not letting the issue go.
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David Letterman Biography

David Letterman Photograph
David Letterman hosts our favorite late-night talk show. His top 10 lists are always good for a laugh. In the summer of 2009, he got into... More »
Full Name
David Letterman