He is crouched over and almost every bone in his back, shoulders and arms are protruding.

The actor looks like an alien, or a starving predator, sniffing around for scraps to eat.

That's just the look Joker director Todd Phillips wanted for Phoenix's portrayal of the great DC Comics villain, The Joker.

"I really feel like he should look like a wolf and look hungry," Phillips told AAP during a recent interview in Los Angeles.

Phoenix lost 24kg in just three months to transform into Arthur Fleck, the sidewalk clown and wannabe Gotham City stand-up comedian who becomes The Joker.

The extreme weight loss enhances Phoenix's stunning performance that has him as one of the early favourites to win the best actor Oscar.

Phoenix, 44, is a three-time Oscar nominee, but is yet to win.

Joker just collected the Golden Lion, the top prize at the Venice Film Festival, boosting its best picture Oscar hopes and vaulting Phillips to one of the directing frontrunners.

Joker is not a superhero movie.

It is a raw, stark and confronting deep-dive origin film exploring how the hapless Fleck became master criminal, The Joker.

"It's a movie that challenges the audience," Phoenix, in a separate interview, said.

"It compels the audience to interact with it in a way that you don't typically find in the superhero genre, in which usually, the motivations of the hero and villain are very clear."

Batman or any other superhero do not make appearances.

You don't see the Joker fall into a vat of acid.

Phillips, Phoenix and other members of the creative team used a "realistic lens" to devise the story.

Fleck has PBA, a condition with uncontrollable laughing and crying outbursts.

His sickly thin body is the result of the medication he takes.

"I don't believe that if you fall into a vat of acid your skin turns white and your hair turns green," Phillips said.

"As much as I love it in the comic books, we thought let's come up with realistic reasons for everything from his laugh, to his look, to his wardrobe."

If Phoenix does win the Oscar for the role, he will follow the footsteps of his late friend, Australian actor Heath Ledger, who posthumously won the supporting actor Academy Award for his Joker performance in 2008's The Dark Knight.

Phoenix and Phillips were full of praise for Ledger's performance, but it had no impact on their portrayal.

"It was pretty important - the interpretation of this character was something that was original," Phoenix said.

Phoenix, after researching the character and medication Fleck likely took, suggested to Phillips he should stack on weight.

"People either put on weight or they lost weight," Phoenix said.

"I thought it would be easier for me to put on weight."

Todd disagreed.

"I had done that before for The Master (2012).

"It was a really difficult process and so I was reluctant to do that again, but ultimately it worked well for the character and informed so much of his movement and physicality and obviously it impacts you emotionally and mentally as well."

Joker is playing in Australian cinemas.

Australian Associated Press