Christopher Nolan is a filmmaker who likes his secrets, so any update on his next film would be huge. But this is genuinely intriguing right here: Variety reports that the Dunkirk director has cast BlackKklansman star John David Washington in his next movie, which is being described only as an "event film." The actor—who happens to be the son of Oscar-winner Denzel Washington—stepped into the spotlight with a role on HBO's Dwayne Johnson vehicle Ballers, but truly broke out thanks to Spike Lee's BlackKklansman. Washington played Ron Stallworth, the real-life African American police officer …