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Finally, some insight into Joaquin Phoenix's bizarre 2008 antics!

A movie chronicling Phoenix's transition from an actor to a rapper is hitting the big screen. I'm Still Here -- directed by Phoenix's brother-in-law, actor Casey Affleck -- opens Sept. 10.

Independent movie distributor Magnolia Pictures insists the flick is not a mockumentary.

Phoenix, 35, had fans scratching their heads two years ago when he began making a series of bizarre public appearances, including growing a scraggly beard and appearing spacey on The Late Show With David Letterman.

At the time, Affleck, 34, filmed all of Phoenix's outbursts, claiming he was shooting a documentary. Many thought Phoenix's antics were just part of an elaborate hoax.

Phoenix was nominated for an Oscar for playing Johnny Cash in 2005's Walk the Line. His last screen role: The 2008 indie drama Two Lovers with Gwyneth Paltrow.