It's no secret that comic book movies are everywhere. Plenty of studios have started their own cinematic universe, with heroes and villains in tights clashing on the big screen, and making tons of money in the process. But there are some adaptations that stand out among the rest, like Todd Phillips' Joker. The 2019 drama was a grueling and thrilling moviegoing experience that made a ton of money at the box office and earned Joaquin Phoenix an Oscar for playing the title character. Phillips assembled a killer cast including Robert De Niro, but it turns out that he and Phoenix clashed for a very specific reason at the start of Joker's production.