Cleese created the comedy troupe with the Life of Brian star, who lost his battle with cancer in 1989, but he now admits he never really understood his friend and found him tough to work with, especially when his well-documented heavy drinking became a real problem.
In a new U.S. TV documentary series, marking the 40th anniversary of Monty Python's Flying Circus, Cleese reveals, "He just didn't work properly... The simple stuff - getting the lines right, hitting his mark, he just was not an efficient creature... He was always late."
Chapman's former partner David Sherlock insists Cleese was "most uncomfortable" around the funnyman after he discovered Chapman was gay.
In the documentary, Month Python, Almost The Truth (The Lawyer's Cut), Sherlock says, "He discovered he'd been working all this time with someone he thought he knew, but now discovered he didn't know."
Cleese admits, "We were all surprised... We didn't mind that he was gay, but we were very, very surprised and I think Graham, sometimes, took the surprise for disapproval."
And Cleese wasn't the only member of the comedy troupe who struggled with Chapman.
Terry Gilliam says, "Graham was just a frustrating person. I never could make out who Graham was."
And Terry Jones adds, "Graham was a mystery... The only times I had lunch with him, we really hadn't got much to say to each other, really."
Michael Palin admits Chapman's drinking problem was a real issue. He recalls, "Sometimes he'd be quite funny and other times... he'd just irritate people for the sake of it."