It would be hard to pick two more fun filmmakers to be around than William Friedkin and Guillermo del Toro. While making his final film, an adaptation of Herman Wouk’s play The Caine Mutiny Court-Martial, insurance mandated that the 87-year-old Friedkin have a shadow on the film who could finish it if he passed away while making it.
Source: What Guillermo del Toro Learned From William Friedkin