While Wayne and others attack the credibility of Arthur's mother Penny Fleck (Frances Conroy) throughout the film, viewers are left to wonder if she is telling the truth. Perhaps Thomas Wayne simply covered up his own misdeeds. Perhaps Batman and Joker really are brothers in this version of the tale.

Cullen, who was surprised when he read the twist, recalls Phillips asking what would be a compelling reason for Joker to hate Batman.

"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 tells The Hollywood Reporter. " This movie makes you feel for Arthur, when you see him struggling with his mother."

Cullen played his scenes as though Penny and Thomas did indeed have an affair, leaving it a strong possibility that he is Arthur's father.

"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 says of his conversations with Phillips. "Later in life, she’s in and out of mental institutions. And in my mind, Thomas Wayne put her there."

In a conversation with THR, Cullen also recalls how the explosive confrontation between Thomas Wayne and Arthur Fleck evolved via rewrites on the fly: "the confrontation was much more comedic compared to the darker original."

How did you get wind of Todd Phillips' Joker film and how were you cast?

Initially, I put myself on tape in Arkansas, where I was filming True Detective at the time. I was in L.A. for a short break and I got a call saying that Todd Phillips wanted to meet. Unfortunately, I was booked to fly to Arkansas the next day.

Not the best timing.

I was fortunate that Todd agreed to meet at 10 a.m. the next day, so I could make my flight. The reading went really well, I was told they were very interested in me for this part. Then I heard that Alec Baldwin had been cast in a mystery role.

You didn’t know he was hired for Thomas Wayne?

No, I kept wondering who he was playing. When I didn’t hear anything, I figured he had my role. Obviously, the studio would love him for Thomas Wayne, he’s an incredible actor.

And then he left over creative differences?

I just know what I read in the trades, which was that Thomas Wayne was going to be played like Trump and Baldwin was not happy and he wasn’t going to do that. I do know that when all that went down, Todd turned to the casting director and said, “Call Brett, that’s who I wanted in the first place.”

What did you find interesting about this interpretation of Thomas Wayne compared to the previous entries in the Batman franchise?

In the other films, he’s always portrayed as very kind and very successful and then he’s dead. (Laughs.) This film delves into Thomas Wayne more, and we tackle the idea that if you’re a successful businessman then there are certain things you do to have that success, and sometimes those things are not pretty…sometimes they’re cutthroat. I think there’s a real edge to him, he’s very much based in reality.

When did you first see Joaquin in character?

I came in for a wardrobe fitting and Todd showed me a scene of Joaquin alone in a bathroom. Nothing was spoken in the scene, but you just felt uncomfortable because it’s so personal. What he communicates through his face and body is just astounding.

What was it like working with Phillips and Phoenix on set?

It was intense, I would call it electric. This wasn’t a film set where people were hanging out, we were working hard and Todd and Joaquin set the tone. Joaquin and I had some great chats in the makeup trailer, but when Joaquin was on set he was in the shoes of Arthur. He lived that part, losing the weight, getting into the spiraling mental state, it was damn impressive.

With all the secrecy surrounding the comic book properties, what access were you given to the script?

On True Detective, we weren’t given the script and there were times when I had no context for my lines. On this film, Todd sent me the script as soon as I was cast.

How did the script evolve as you shot it?

We shot several scenes different ways because Todd wasn’t sure whether some scenes were too dark or if we might need a lighter moment. Todd’s vision is truly unique and I wouldn't be surprised if the film and Joaquin are up for Oscars.

In regard to the film’s powerful revelation, what was your initial reaction and did you discuss it with Todd?

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.

What was the relationship in your mind between Thomas Wayne and Arthur’s mother?

I asked Todd how Thomas Wayne would’ve know 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.

What was the most challenging scene to shoot?

Without a doubt, it was the confrontation between Arthur and Thomas Wayne. We shot that a couple different ways. That was a challenging day, especially for Joaquin who had to be very emotionally dialed in to his character.

I remember at one point the cameras were on me and Joaquin was off camera. He started rubbing his face, then he walked in a circle, and then left the set. I thought maybe I’d done something, but Todd said, “No that’s just how he is. Something was bugging him and he needs to reset.”

There were moments that day when we threw out the dialogue and just riffed which was great, almost out of necessity. That scene was rewritten and delivered to me at 3 a.m. via email. I’m big on preparation and this was a very big scene, so I jumped right into it. Then when I got to makeup that morning, they gave me a completely new version of the scene. And then as I walked to set, they gave me a newer draft and told me Todd and Joaquin were ready to rehearse with me.

I told them, “Guys, I don’t know these pages at all.” Joaquin laughed and said, “It’s OK, neither do I. By the time we shoot we’ll probably have it.” I really relaxed at that point. The version of the confrontation was much more comedic compared to the darker original.

What do you hope audiences take away from this film?

I think the film is about how a man can spiral, and how the way he’s treated in life influences what he ultimately becomes. If you look at how the government and VA treats our veterans, it’s nowhere near the level of care they deserve. I read about a veteran being found in a VA hospital covered in ants. These guys sacrifice for our country and very few of them get the treatment they need. There’s the conversation in the mental health world, where prescribers think putting people on drugs will just fix everything. People need more care and not to be cast aside.

Tim Burton’s Batman (1989) launched a billion-dollar film franchise for Warner Bros. What was your reaction to the Burton-Keaton Batman and the films leading into Joker?

I went to University with Dennis Quaid and Robert Wuhl, so when Robert got Batman film, we were all excited for him. That film was certainly unlike anything we’d seen before in cinemas. I don’t think you can have a conversation about Batman without mentioning Christopher Nolan and his Dark Knight trilogy.

You worked with Nolan on Dark Knight Rises. What was that process like for you? Were you concerned it might limit your chances at future DC roles?

When I went into the audition, I was surprised how fast actors were going in and out of the room. Chris doesn’t like to waste time and we got right to work. He told me to think Bill Clinton for this Senator role, and three weeks later I got the part. I didn’t think about how it might affect future DC film roles. In fact, I got sent an audition for Person of Interest because of Dark Night Rises. Jonah Nolan was watching the dailies and was impressed enough to fly me to New York for an audition. So, I actually booked more work because of it.