VIDEO: Johnny Depp Impersonates Al Pacino on 'David Letterman'

Johnny Depp worked with Al Pacino on Donnie Brasco and told David Letterman last night on The Late Show With David Letterman that, while Al is fantastic to work with on set, he didn't know he was "certifiably insane."

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"Al's fantastic, I mean Al's a legend. He's Al Pacino. He's an amazing actor," Johnny told David. "The one thing I didn't really know about Pacino prior to shooting was that he's certifiably insane."

"He's a joker, he likes to tell jokes and he liked to tell me this one specific joke over and over and I really didn't understand it," Johnny explained. "I didn't get the joke."

After The Tourist star said the joke Al told him over and over again, David admitted, "I must say, you kind of expect more from Al Pacino."

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Watch the interview below!