The leading man also discusses what drew him to the iconic role.
For Joaquin Phoenix, becoming the Clown Prince of Crime involved a lot more than makeup and a colorful suit.
The acclaimed leading man sat down with ET's Ash Crossan to discuss losing over 50 pounds in order to take on the titular character in DC's upcoming origin film, Joker, and how the transformation informed his performance. Phoenix said that he began by researching the side effects of medication on patients.
"And one thing that everybody was talking about on these [message] boards was how their medications affected their weight. And some people would talk about losing a lot of weight and some would talk about gaining weight," he explains. "And so, I said, 'Let's go with the weight gain' because that would be easier for me. And Todd [Phillips, the film's director] said, 'No, no, no, you are going through weight loss.'"
"And so, it was something that I've done before and… I swore when I did it before that I'd never do it again. But, it seemed like it was very important for the character and I think it ended up affecting me in ways that I hadn't anticipated," the 44-year-old leading man added.
ET also spoke with Phillips, who shared more about Phoenix's weight loss for the role and how he convinced him that his character ought to be lean.
"I think we was looking for the easy way out at that point," Phillips jokes of Phoenix's desire to gain weight for the film. "He said, 'You know, I think he should be heavy, we really haven't seen a heavy Joker,' and I said, 'I get it, but I really believe that Arthur needs to look hungry and wolf-like and malnourished. It was bit of a conversation and then he just started losing the weight and he lost, I think, 52 pounds. It was crazy."
While speaking with Phoenix, he also explained that, after shedding all those pounds, he discovered a new kind of freedom in terms of body movement, which he said was significant in how the character moves in the film.
"Certainly, in respects to your emotional and mental state. But, really, the thing that I didn't expect at all was how I felt I could move my body once I'd lost all that weight," the actor shares. "I felt this unique control over my body and how I moved, for whatever reason."
Phoenix also shed some light on why this comic book character was the first he decided to tackle, noting the difficulty that comes with a role like the Joker.
"Because it was complex in a way that other movies I flirted with weren't," he states. "And so, I thought that it was gonna be challenging for me as an actor. I thought it was gonna be challenging for the audience. And that's exciting to me."
"I think I always look for characters that aren't easy to define," he adds. "And they're surprising in many ways. And so that's what this was. You hear Joker and I think anybody has their idea of what they think the film will be. I think Todd and Scott Silver's script really went ways that I couldn't have expected."
In a few scenes in the film, the Joker showcases his maniacal dancing skills, which Phoenix explained took some work with a choreographer, as well as pinpointing where that desire to dance came from for this anti-hero.
"Joker has this certain kind of arrogance and this need to be revered and adored," the actor shares of the Joker's grandiose dancing. "And he sees himself as a king in some ways."
Joker arrives in theaters on Oct. 4.
See more updates on the film below.