Endlessly fascinating casting decisions, part one

Jessica Winter's review of The Upside of Anger, a new movie directed and written by Mike Binder, reveals that Binder cast himself as the guy who has lots of sex with Erika Christensen. I wonder if, on the opening pages of the script, the character Binder portrays weren't actually described as "Guy who has lots of sex with [Character X]" while Character X is described as "hot twenty-ish girl I want to grope and dampen with my saliva." I imagine the search for an actor to fill the part Binder took was probably the most ardent quest the world has seen since W asked Dick Cheney to find him a VP.