Major spoilers for Joker follow below.

While Joker was billed as a standalone, wholly separate version of the titular DC Comics villain, the film makes some surprising revelations with regards to other famous DC characters—namely Bruce Wayne. But given the ambiguity with which the film ends, are we to take these revelations as fact? Actor Brett Cullen, who plays Thomas Wayne in the record-breaking film, has some insight.

Joker tells the story of a lonely, mentally ill man named Arthur Fleck (Joaquin Phoenix) who slowly spirals into more disturbing and violent behavior, until he eventually takes on the moniker of Joker and inspires chaotic violence all across the streets of Gotham City with one publicly violent act: he shoots and kills TV host Murray Franklin (Robert De Niro) live on air.

Key to Arthur’s spiral is new information he learns from his mother Penny (Frances Conroy), with whom he lives. Arthur finds a letter Penny has written to Thomas Wayne, claiming that Wayne is Arthur’s real father. When Arthur confronts his mother about this information, she claims that she worked for Thomas Wayne, “a kind man,” she says, and the two had an affair. But when she had Arthur, Thomas—a public figure with political aspirations—took steps to hide the legitimacy of his fatherhood.

Arthur eventually confronts Thomas, who says Penny is lying. He claims she’s a mentally unstable woman who had a fixation on him, and that he’s not Arthur’s father. Arthur then tracks down his mother’s medical records at Arkham Asylum, which notes that he was abandoned as a child and adopted by Penny, whose abusive boyfriend physically harmed both herself and Arthur, causing severe trauma. The records also claim that Penny suffers from psychosis and compulsive lying—although Penny later claims that Thomas falsified these records to further hide the truth of his illegitimate son.

Joker is a film that delights in blurring the line between fiction and reality, as we also later learn that the relationship we’ve witnessed Arthur striking up with his neighbor Sophie (Zazie Beetz) was all in his head. Moreover, the final scene of the film throws the entire movie into question, as it presents the possibility that we’ve simply witnessed one long “joke” the Joker has been telling to a mental health professional while locked in Arkham Asylum. A “multiple choice” origin story, if you will.

So what’s real and what isn’t? Is Joker really Batman’s brother? Cullen spoke with THR and revealed that when he played his big bathroom confrontation scene with Phoenix, he played it as though Thomas Wayne is actually Arthur’s father:

“I was very surprised when I read the twist. I went to Todd and asked, ‘Are we playing this the way I think we’re playing this?’ Todd responded with, ‘What would be a compelling reason for the Joker to hate Batman so much?’ 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.  This movie makes you feel for Arthur, when you see him struggling with his mother. And she’s saying, ‘Go see Thomas Wayne, he’ll help us. He’s a good man.’ It’s gut-wrenching.”

Indeed, Cullen says he buys into the idea that Thomas Wayne irreparably ruined Penny’s life after she became pregnant:

“I asked Todd how Thomas Wayne would’ve known Arthur’s mother. 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. Later in life, she’s in and out of mental institutions. And in my mind, Thomas Wayne put her there.

 

What I like about the film is that it’s about real people with real faults who make mistakes. Some are done out of protection, like moving Arthur and his mother out of the picture.”

This certainly does give Joker motivation to become Batman’s arch-nemesis whenever the young Bruce Wayne eventually dons the cape and cowl, and it’s an interesting addition to the many different iterations of Joker’s origin story that have been told across different mediums. With The Dark Knight, filmmaker Christopher Nolan opted to keep the Joker’s origins shrouded in mystery, always changing. But Joker is kind of the antithesis of that idea, as the big twist reveals a very real and long-lasting connection between the two iconic comics character.

Or maybe it doesn’t. While Cullen certainly believes Arthur is Bruce’s half-brother, Phillips takes great pains to make sure the audience questions all the information they’re given, and the film itself doesn’t definitively come down on one side. So, much like the Joker in The Killing Joke, audiences are given a multiple choice answer. You can believe Arthur truly is Bruce Wayne’s half-brother and thus now has a reason to hate him for the rest of his life, or you can believe that Penny was crazy, and Arthur/Joker merely believes he’s Batman’s brother. Either way, the Clown Prince of Crime is no fan of Baby Bruce, and should a sequel actually come to fruition, that'd be an interesting dynamic to see play out.

For more on Joker check out our breakdown of that ending, which similarly leaves the big answers in the audience’s hands.