I wasn't sure I could pull off playing the Joker, says Joaquin Phoenix

Joaquin Phoenix, who famously starred in hits Gladiator and Her, to name just two of his many starring roles, is a conundrum.

While interviewing him I got the feeling that something unexpected was going to happen. I took time to frame my questions carefully. If I didn't phrase them correctly, I got the sense that verbal combat could ensue.

I say "feeling", because it probably wouldn't. He's probably perfectly amiable - but his eyes had the power to unnerve me. Colourless glass beads that speak volumes, while his mouth opens with first responses, edited as he looks at me, brain ticking over, jet eyebrows raised, contrasting with almost transparent irises.

When I read back over the conversation, I see that he's been sweet all along. Still, there's the illusion that at any moment he could be other than sweet, unpredictable.

It's that tightrope that's highlighted on screen as his face spreads, gigantic and up-close across it. Pathos. You feel like you want to protect him - though he probably doesn't need your protection.

In director Todd Phillips's much-anticipated Joker, he gives full birth to that vulnerability so you want to embrace - and then run - from him, with equal intensity.

As the movie progresses, the urge to run becomes stronger. It's an exquisite performance - the origin story of how Arthur Fleck becomes the Joker. Phoenix dominates every shot, giving a calibrated character study that makes you root for him even as you're repulsed by his character. And repulsed you are.

Phoenix is the middle child of five, and brother to Summer and deceased actor River.

He says of his character in Joker: "There's a part of me that sympathises with him, but I also find him despicable. That's what I liked about it."

Phoenix has won a Golden Globe and a Grammy - both for Walk the Line. He's also received three Academy Award nominations, for Gladiator, Walk the Line and The Master.

"My feelings about the character are complex," he says.

"I hope that I can empathise and understand what's it like to be him, but there are other times when I think, you should take f**king responsibility as well.

"This is not somebody who is purely innocent even from the beginning. It's somebody who's a textbook narcissist. He demands that the world not only recognise him and want to hear his voice, but totally adore him too. I find those traits unbecoming, but that's what I liked about playing this character - there isn't an easy answer for any of the questions that the movie poses.

"I'm not in the camp of people who have total sympathy for him, those who feel that if somebody had just been nice to him everything would have been OK. I don't think that's the case."

Despite the juicy role, Phoenix didn't jump at the chance to play Batman's nemesis. "I've been struggling with decisions for the last few years," he says. "I've been taking more time to make up my mind. It was a long process. Mostly, I wasn't certain that it was something I could pull off."

He gives a disarming grin when asked why he struggled. "I don't know why I said that, honestly. I don't know that I struggled. I think I went back and forth on whether I wanted to do it or not over the course of a couple of months. I used to make decisions right away."

He shrugs. "There's a lot to figure out. The biggest question when you go on a journey with any filmmaker is: are we compatible? Sometimes it's like, do you want to hang out with the person for f**king three months?

"Some of it is really that simple. There are some people that elicit the reaction: 'I don't think I could be around you longer than two weeks. How am I going to work with you?' So part of it is: do we like each other? Do we share the same sensibilities? Do we like the same kinds of films? Are we trying to say the same thing? And you want to challenge each other. Maybe that's what that underlying feeling encouraging me to be careful is all about," he says.

Phoenix, who began his career calling himself Leaf, likes to challenge. It's not necessarily confrontational, but he will challenge you on things that others might let slide.

He elaborates: "You don't want to be completely in step. It's good to have opposing energy that makes you push other people … and get pushed. I've been in a couple of situations where I didn't get along with the director.

"There was no struggle this time. I knew we were in sync the second time I met Todd. The first time was for 10 minutes when he gave me the script. The second time, we got together and talked, and I realised I really love this guy."

He confesses to sometimes being torn when it comes to taking on a role: "Do I really want to go away and work, or should I hang out with family?" Phoenix is engaged to actress Rooney Mara.

In this case, "getting Phoenix to say yes took a couple of months", says the director. "He's done a lot of low-profile movies for the past few years because he prefers being under the radar and doing great work. He knew this wasn't going to be under the radar, so I think he was careful about committing."

Phoenix debunks the idea of being affected negatively by the characters he portrays. "Maybe that's true for others. When I hear other actors say, 'I had nightmares,' like, that sounds so cool. They were really committed. But I've got to be honest, I don't have that experience. Maybe it's because I've been doing this since I was eight years old.

"Honestly, I had more fun than I've ever had on any movie, making this. I've never laughed so much as I have working on this movie. Todd is the funniest guy."

Phoenix lost 24kg for the role, choosing not to go on diet until the last minute and then restricting himself to an apple a day. Just one - and sometimes a salad without dressing. That could make anyone go a bit crazy.

Phillips points out that Phoenix had some padding before his drastic weight loss. "He was fat when we started - bulky. He needed to lose [7kg]. We wanted Arthur to look sickly; wolf-like, hungry. I kept saying, 'Start now', and he'd say 'That's not how I do it. I'll start, I'll start.' From June 1 to September 1 he lost all that weight. He went from [82kg to 58kg]. It was ridiculous."

When the topic is raised with Phoenix he claims he doesn't remember his first meal after the gruelling diet. "It probably was food in an airport, sadly," he says. He gives a lopsided grin. "I think that I just ..." His voice trails off. "You get to a state where you eat a stalk of celery and can taste seven different flavours."

• 'Joker' opened in cinemas in SA on October 4.

JOAQUIN PHOENIX'S ON SCREEN HISTORY

1974: Joaquin Phoenix was born Joaquin Rafael Bottom on October 28.

1986: Calling himself Leaf, he started acting in television series with his brother River and sister Summer and released his first major film, SpaceCamp, when he was 12 years old.

1995: After going back to his given first name, he achieved the first of his big successes with the portrayal of murderous delinquent Jimmy Emmett in the criminal comedy To Die For.

2000: For his portrayal of the evil Commodus, Marcus Aurelius's wayward son in the film Gladiator, he earned a nomination for Best Supporting Actor from the Academy Awards.

2005: Next up was a nomination for Best Actor for his portrayal of musician Johnny Cash in the biopic Walk the Line alongside Reese Witherspoon, who played June Carter (she won the Best Actress award for the role). Phoenix recorded the soundtrack to Walk the Line, for which he won the Grammy Award for Best Compilation Soundtrack for Visual Media.

2012: Phoenix plays a sex-obsessed alcoholic war veteran in The Master, a dramatic film about the birth of Scientology for which he won his third Academy Award nomination.

2013: He is lauded for his portrayal of Theodore Twombly, a man who develops a relationship with Samantha (Scarlett Johansson), an artificially intelligent virtual assistant in the Spike Jonze produced film Her. It received five nominations at the 86th Academy Awards, including Best Picture, and won the award for Best Original Screenplay.

2017: Phoenix wins the Cannes Film Festival Award for Best Actor for his role as Joe, a traumatised hired gun in the existential thriller You Were Never Really Here.