Joaquin Phoenix was hesitant about taking on the role of Joker.

The 44-year-old actor plays the mentally ill stand-up comedian in the upcoming origin 'Joker' film and he has admitted, in hindsight, he was probably filled with a ''motivating fear'' to play the part just right.

He told Total Film magazine: ''It took me a while [to commit]. Now, when I look back, I don't understand why... There was a lot of fear, yeah.

''But I always say there's motivating fear and debilitating fear.

''There's the fear where you cannot make a f***ing step, and there's the kind where it's like, 'OK, what do we do? That's not good enough.'

''And you're digging deeper and deeper. I love that kind of fear. It guides us, makes us work harder.''

Meanwhile, the 'Signs' star recently said he fears that the upcoming movie based on the DC Comics villain may not live up to fans expectations as he and director Todd Phillips chose to stray from the original material and focus on the ''story of becoming Joker''.

He said: ''We didn't follow anything from the comic books, which people are gonna be mad about. We just wrote our own version of where a guy like Joker might come from.

''That's what was interesting to me. We're not even doing Joker, but the story of becoming Joker. It's about this man. ''

The 'Gladiator' actor added that he thought the project sounded ''super ambitious'' when he first discussed it with Phillips.

He explained: ''I thought it was crazy-ambitious. Todd said to me 'I have this idea, I wanna tell the origin story of the Joker. I said 'Wow okay'''.