EXCLUSIVE: Casey Affleck has completed that much-talked about and long-awaited mockumentary he directed about his brother-in-law Joaquin Phoenix’s eccentric metamorphosis from actor to hip-hop musician (or so we’ve been led to believe). I’m told that the film made its debut in a private lunchtime screening at WME headquarters last week for buyers — including Harvey Weinstein — who were sworn to secrecy. WME is selling the film, and it may only take a couple of days to reach a deal. I hear the agency and the distributors intend to keep the mock’s content under wraps for as long as they can for maximum shock value. (So it won’t be part WME Global’s sales push at the Cannes Film Festival.) Presumably, the film answers Hollywood’s bewilderment about whether Joaquin was serious about quitting acting — or whether he was just, well, acting. His long months of bizarre behavior culminated in his semi-mute interview with David Letterman that was as surreal as anything Andy Kaufman ever did. And, his musical debut at the Las Vegas night club LAVO where he claimed to be a homeless rapper, jumped around the stage, and fell off.