Kevin Spacey [1] has already proven himself to be a master of celebrity impressions [2], and during an appearance on Late Show With David Letterman on Monday, he pulled out all the stops. The actor started the show off with a bang by impersonating his mentor and friend Jack Lemmon before revealing that he met Jack when he was just 13. Kevin then followed up with a Johnny Carson monologue that only he could pull off so well. Watch the clip above, and then check out Christina Aguilera [3]'s hilariously spot-on Britney Spears impression [4].