Sir Ridley Scott cast Joaquin Phoenix in 'Napoleon' after an inspirational moment while watching 'Joker'.

The 85-year-old director is helming the upcoming historical epic that sees Phoenix play the role of the French Emperor, and Scott knew the 48-year-old star was the man for the role when he was watching him play the titular character in the 2019 film.

He told Empire magazine: "I’m staring at Joaquin and saying, 'This little demon is Napoleon Bonaparte.'

"He looks like him."

Scott compares Napoleon to Nazi Party leader Adolf Hitler, but says he was "extraordinary".

He added: "I compare him with Alexander The Great. Adolf Hitler. Stalin.

"Listen, he’s got a lot of bad s*** under his belt.

"At the same time, he was remarkable with his courage, and in his can-do, and in his dominance. He was extraordinary."

Scott and Phoenix previously teamed up in 2000 historical drama 'Gladiator', and the actor admits he had a feeling of nostalgia about working with the director again for 'Napoleon'.

He added: "There was just a very nostalgic idea of working with Ridley again.

"I was so young. It was my first big production. I really yearned for that experience again, or something similar."

In December, Scott told how he rewrote 'Napoleon' because of Joaquin Phoenix's "constant questioning".

Speaking to the same publication, he said: "Joaquin is about as far from conventional as you can get.

"Not deliberately, but out of intuition. That's what makes him tick. If something bothers him, he'll let you know.

"He made ('Napoleon') special by constantly questioning.

"With Joaquin, we can rewrite the goddamn film because he's uncomfortable. And that kind of happened with 'Napoleon'.

"We unpicked the film to help him focus on who Bonaparte was.

"I had to respect that, because what was being said was incredibly constructive. It made it all grow bigger and better."