Hunched over a table on the patio of a downtown Toronto hotel, Joaquin Phoenix starts to squirm when I ask him if he knew what he was getting into when he agreed to star in Joker, an origin story that recounts the rise of the famed Batman nemesis.

“Two days before we started shooting, I came here for The Sisters Brothers. I was doing press in Toronto for that film and during those interviews I was asked repeatedly about Joker and I suddenly realized that this held a lot of weight for many people.

“I remember one journalist saying to me, ‘People are really anticipating this and how do you feel about that?’ and I kind of just looked at him and said, ‘Why are you telling me this?’ ”

But Phoenix — who has eschewed high-profile movies in favour of smaller character-driven pictures revolving around loners and outsiders, including The MasterHer and last year’s You Were Never Really Here — was intrigued by the idea of exploring a possible origin of the comic book criminal as imagined by co-writer and director Todd Phillips (The Hangover trilogy).

“There’s something about a Todd Phillips movie that makes his films unique. So I knew that I wanted to work with him,” he says.

The three-time Oscar nominee, who has also played Johnny Cash (Walk the Line) and Jesus of Nazareth (Mary Magdalene), says his interest was further piqued by the idea of doing a standalone comic book movie that wasn’t tethered to DC’s ongoing Extended Universe, which is currently led by Superman, Wonder Woman, Aquaman, Harley Quinn and Shazam!

Created by Bill Finger, Bob Kane and Jerry Robinson in 1940, the supervillain doesn’t have a defined origin tale. One popular version sees the character making his nefarious emergence after falling into a vat of acid. But Phoenix says he liked how this new reimagining was grounded in the real world.

Phillips’ 1981-set story casts Phoenix as Arthur Fleck, a gaunt street clown and a failed stand-up comic in Gotham City who slowly becomes unglued as he grapples with mental illness and his ailing mother’s hopes to insinuate herself back into the lives of her former employers, the Wayne family. Arthur’s grip on reality is further loosened as he begins to obsess about getting a chance to try out his act on a variety show led by a cocky late-night host (played by Robert De Niro).

Phoenix, 44, follows a long list of actors who have each put their own stamp on the Joker, including Cesar Romero, Jack Nicholson, Heath Ledger (who won a posthumous Oscar for his take) and Jared Leto. But he says he didn’t study any of those prior performances.

“I didn’t want to be influenced by anything I might see,” he says slinking back into his chair. “I want why I’m doing a film to be motivated by me personally and how I’m responding to the subject.”

To prepare for the role, Phoenix, who credited his career to his late brother River while accepting the TIFF Tribute Actor Award in Toronto earlier this month, lost more than 50 pounds and delved deep into researching the effects of mental illness.

“What happened for me is (the character) just started presenting himself.”

Just days after Joker took home the Best Film Prize at the Venice Film Festival, a relaxed-looking Phoenix spoke about stepping into the role and whether his take on the Clown Prince of Crime really is a one-off.