WARNING: The following contains spoilers for director Todd Phillips' Joker, in theaters now.
In Todd Phillips' Joker, one key revelation that led to the downward spiral of Joaquin Phoenix's Arthur "Happy" Fleck into this version of the Clown Prince of Crime was that he might have been the illegitimate son of Brett Cullen's Thomas Wayne in the movie. Now, according to Cullen himself, we find out exactly what's the deal behind this plot twist.
"The idea that the Joker is an illegitimate child that didn't get anything from the Wayne family is a very compelling motivation for his character's hatred," Cullen said to The Hollywood Reporter. "This movie makes you feel for Arthur, when you see him struggling with his mother [Penny]."
"The backstory was that Arthur's mother had worked for Thomas in his home, and she was a beautiful woman who Thomas was attracted to and it led to a physical relationship," Cullen added on his discussions with Phillips on his character's motivations. "Later in life, she's in and out of mental institutions. And in my mind, Thomas Wayne put her there," he continued.
By the time the movie wraps, it remains vague if he really was Arthur's father, because while Thomas and Alfred denied this, claiming it was all delusion due to Penny's waning mental health, we did see a photograph Arthur uncovered that had an affectionate signature from the elder Wayne. This hinted he was indeed covering up an affair, ignoring Penny's requests for aid in her impoverished state and even going so far as concocting a fake story about how she adopted Arthur.
Directed by Todd Phillips, Joker stars Joaquin Phoenix, Robert De Niro, Zazie Beetz, Bill Camp, Frances Conroy, Brett Cullen, Glenn Fleshler, Douglas Hodge, Marc Maron, Josh Pais and Shea Whigham.