Phoenix negated being influenced by past silver screen Jokers. He lost 23 kgs (52 pounds) for the part, fully inhabiting his own version of the character which first appeared in the original Batman comic in 1940, reports hollywoodreporter.com.
Losing weight in such a short time helped Phoenix get into his character.
"It turns out that affects your psychology. You start to go mad," he said.
Director Todd Phillips also gave him a blank journal/joke book which helped him uncover his character as he began to fill in the pages, with words and images that are seen in the film.
In "Joker", Phoenix stars as Arthur Fleck, a man living in near-poverty with his mother (Frances Conroy), struggling to make ends meet as a clown for hire.
Affected by a brain trauma, Fleck served time in a mental ward, is treated by a listless city social worker who oversees his seven different medications, and struggles with a condition that causes him pathological laughter at the most inopportune times.
"He's so hard to define and you don't really want to define him," said Phoenix.
"We would get close at times where I found that I would identify certain parts of his personality or his motivation and then I would back away from that because I wanted there to be a mystery to the character. Throughout the course of shooting it felt like every day we were discovering new parts of his personality, up until the very last day," he added.
Phoenix also went in-depth in how he developed his signature laugh for the role, saying it needed to be "something that's almost painful. I think for Joker it's a part of him that wants to emerge. I think we all kind of assume what a Joker laugh is and it felt like a new, fresh way of looking at it."
The laugh itself was many months in the making. "I didn't think that I could do it. I kind of practiced alone but I asked Todd to come over to audition my laugh. I felt like I had to be able to do it on the spot and in front of somebody else. It was really uncomfortable. It took me a long time."
( With inputs from IANS )