O n a recent Tuesday afternoon, Johnny Knoxville stood on a concrete retaining wall and surveyed the expansive scene before him. There were smatterings of desert bloom. There were patches of rocky soil and scruffy brush. There were tantalizing pockets of cactuses. But mostly, there was a vertiginous drop-off, the steepest of slopes careening down to six-lane La Brea Avenue, where Los Angeles traffic zoomed by at an unholy clip. It was, in other words, a tempting locale in which to insert a human being into a barrel and roll said barrel down the hill. Or, better yet, a porta-potty! An inflatable sumo-wrestler suit? A human hamster wheel? The possibilities were endless, but— “Face me a little more,” instructed a photographer, perhaps interrupting Knoxville’s reverie. “Let the sunlight come on your eyes.” “Uh, you had me at ‘Cum on my eyes!’ ” Knoxville replied, turning away from the danger and flashing that slightly unhinged, crocodile grin.…