Sporting a black suit paired with a light blue button-up and a black tie, Cooper cut a distinguishing figure while on hand to promote the comic book adaptation. Although his appearance was a surprise to fans, the 44-year-old leading man wasn't merely there to show his support -- he's a producer on the project. He and director Todd Phillips have worked together on a number of projects, beginning with 2009's The Hangover.
Besides Phillips and Phoenix, the American Sniper star also posed with veteran actor Robert De Niro, who has a supporting role in the film as a talk show host.
The film premiered at the Venice Film Festival in August, where it earned the Golden Lion jury award and sweeping acclaim for Phoenix's portrayal of villainous titular character.
ET's Keltie Knight was on the red carpet in Toronto, where she spoke with Phillips about the ambitious project and its leading actor.
"It was a dream," he gushed of winning the award in Venice. "Joaquin is one of the greats and when he digs into a character, he does it like nobody else and he just went for it."
However, when asked if there's any possibility that Phoenix's Arthur Fleck/Joker might someday cross paths with Robert Pattinson's Batman, he said there was no chance: "I'll be very clear. Zero percent."
Frances Conroy, who plays Fleck's mother, also chatted with ET about what makes the project so special for her.
"Because it's about humanity. It's about a person. It's all real. It's just life. You see a slice of life that will stay with you for a long time," the 65-year-old actress said. While discussing Phoenix's work in the film, she added, "He's a great actor and gives a magnificent performance. He's transcendent."
Joker laughs its way into theaters on Oct. 4.
See more updates on the film below.