Matt Damon was the gullible victim of a wicked practical joke eight years ago. At the American Cinematheque gala in LA, Casey Affleck — brother of Damon’s best friend and fellow Oscar winner Ben Affleck — related how he and Damon were co-starring in London in “This Is Our Youth,” when Damon had to return to the US for two weeks, leaving his understudy to play his role. Upon his return, Damon took the stage at the Garrick Theatre. It was strangely quiet as the curtain opened. “There was absolutely nobody in the audience,” Casey told the Beverly Hilton crowd including Clint Eastwood, Greg Kinnear and Charlize Theron. As reported by Montecito Journal columnist Richard Mineards, Affleck explained, “We thought we’d play a big joke on Matt.” But Affleck said Damon gamely carried on with his role for the whole play: “When I asked him why he continued, despite the seats all being clearly empty, he said he thought we were doing a private viewing for the Queen!”