In a 2007 interview, the legendary actor was asked by MTV how he felt about Ledger landing the role.
"Let me be the way I'm not in interviews, I'm furious," he said
"They never asked me about a sequel with the Joker. I know how to do that! Nobody ever asked me ... Maybe it's not a mistake. Maybe it was the right thing, but to be candid, I'm furious."
When asked why he was so fired up, Nicholson said: "Well, the Joker comes from my childhood. That's how I got involved with it in the first place. It's a part I always thought I should play."
Jack Nicholson as the Joker and Michael Keaton as Batman in 1989's Batman. Photo / News Corp Australia
Matt Damon turned down a role in the film
The Jason Bourne star was asked to play Harvey Dent in The Dark Knight but had to turn it down.
"I couldn't - there was a scheduling thing," Damon said to MTV.
But the Hollywood nice guy added that he doesn't think he could have done better than Aaron Eckhart who ended up playing Gotham City's district attorney.
"Look, Aaron is a great actor, so the movie didn't suffer for it. Every now and then you get one and you can't do it."
Aaron Eckhart as Harvey Dent in The Dark Knight.
Ledger asked to be beaten
If the fight between Batman and the Joker in the interrogation rooms looks realistic, that's because it was.
Bale told The Independent that Ledger actually encouraged him to be rough.
"In the movie, Batman starts beating The Joker and realises that this is not your ordinary foe," Bale said.
"The more I beat him, the more he enjoys it; and Heath was kinda egging me on, going, 'Go on, go on, go on!' But he was slamming himself around, and there were tiled walls inside of that set which were cracked and dented. His commitment was total."
Speaking about the same scene, director Christopher Nolan said to Hero Complex: "They had rehearsed the stunts and the fight stuff very specifically, but we really let the actors work within that. I had never seen anybody sell a punch the way Heath was able to with Christian. I got the violence I wanted."
Ledger was tossed around like a salad in this scene.
Batman never rode the batpod
The Batpod, which is basically an anti-aircraft gun on two wheels, is so difficult to drive that only one man was allowed to get behind the wheel.
"The guy who did ride it in the film, Jean-Pierre Goy, was the only guy, out of dozens of bike champs and stunt men on set, who could actually stay on the damn thing for any length of time," Bale said to GQ.
"Embarrassingly, I didn't get to ride it. There are other motorbikes in the film that I got to burn about on, but not the Batpod - it was deemed too dangerous; they needed me in one piece to finish the damn movie."
That's not actually Bale on the bike.
Political cameo
When the Joker gatecrashes a party in The Dark Knight, a man stands up to him and says, "We're not intimidated by thugs".
The Joker responds by saying, "You know, you remind me of my father. I hated my father".
It turns out that man is actually US Senator Patrick Leahy who is such a DC Comics fan that he's appeared in five different Batman movies.
"He (Heath Ledger) scared the heck out of me with the knife," Leahy said to Roll Call, "I didn't have to act."
The Senator from Vermont was extremely impressed with Ledger's performance on set: "We'd be sitting around the chow line or whatnot and he's just joking with everybody, then they'd say 'Action,' he'd turn into this, very, very menacing person."
Patrick Leahy also appeared in this year's Batman v Superman: Dawn of Justice.
Dubbed like Dad
When Batman was released in 1989, an actor called Giancarlo Giannini dubbed Jack Nicholson's voice for the Italian version of the movie.
Fast forward to 19 years later and it was his son, Adriano Giannini, who dubbed the voice of Heath Ledger's Joker for the Italian version of The Dark Knight.
Fun fact: Adriano Giannini co-starred with Madonna in the rather forgettable 2002 movie, Swept Away.
Was the real hero actually The Joker?
There's a fan theory that perhaps Heath Ledger's character isn't as evil as everyone thinks.
"Before the Joker, Gotham was a mess," wrote Reddit user generalzee.
"Entire sections of the city were closed off due to madness, organised crime ran rampant, and the majority of important city officials were wildly corrupt. The city even tolerated a renegade vigilante who ran around wearing a rubber suit.
"Along comes the Joker and by the end of a very short time, almost all organised crime was eliminated, many corrupt officials were imprisoned or dead, and the city's vigilante even went into hiding for eight years. This was all part of Joker's masterfully executed plan."
Heath Ledger won an Oscar for his role as the Joker.
OK, so that's the theory, but where's the evidence?
"The very first thing we see Joker do is rob a mafia controlled bank, eliminating the entire team of expert bank robbers who helped him pull it off," wrote generalzee.
The Joker then begins "murdering key criminals and corrupt officials that could help insulate those at the top".
"Gordon's promotion, too, did a major service to Gotham. I think a lot of people take the Joker's clapping during Gordon's promotion scene to be sarcastic, but I actually think that Joker believed in Gordon, one of the few officers on the force who was truly incorruptible."
In conclusion, the Reddit user argued that "Gotham was safe because the Joker had cleaned up the streets".
"He eliminated the corrupt police, he destroyed organised crime financially, he uplifted Gotham's spirit, and he even got rid of the flying pest that had been corrupting Gotham ever since he declared himself its protector."
