Christopher Nolan reflects on Heath Ledger's Joker in The Dark Knight on the 10th anniversary of the actor's death. Ledger passed away on January 22, 2008, from a lethal combination of prescription drugs. His untimely death shocked the industry, especially those who worked alongside him on Nolan's DC film, as well as all of his fans around the world. Although his death cast a shadow on the movie's production, it actually ended up propelling The Dark Knight forward at the box office.

The Dark Knight was an unprecedented success, both critically and commercially. The film became the first superhero movie to gross more than $1 billion at the worldwide box office, partially because it was the first movie ever to be filmed with IMAX cameras. In fact, aside from 2013's Iron Man 3, The Dark Knight and The Dark Knight Rises are the only other solo superhero movies ever to cross the billion-dollar mark. But that's not all, The Dark Knight also scored a whopping eight Oscar nominations. And now, almost 10 years later, Nolan reflects on the making of the film.

Related: The Dark Knight Returning To Some Theaters For 10th Anniversary

In an interview with BBC Radio 1 that published on the 10th anniversary of Heath Ledger's death, Christopher Nolan briefly looked back on Ledger's iconic portrayal of the Joker, starting with Ledger's proclivity for improvising on set.

"...He would sort of give me hints about what he was going to do. We would talk about it a bit. And I would try and be an audience for him and sort of engage with him, what he was doing. But a lot of it was about unpredictability, and I think he wanted to play his cards close to the chest. He would very gradually reveal to me the 'voice' and the way he was going to do things - but not in one go, like, 'Here's the Joker.'

"We watched him sort of develop it, with the wardrobe and the makeup, and I kind of got to be a part of that creative process, which was great fun, but on-set, there were always moments like that clapping or things he would do with his voice. His voice was so unpredictable. He created this bizarre pitch. I've seen a lot of people try and imitate it since. But we never quite knew if he was going to go high or if he was going to go low. You never knew what that guy was going to do, and that's what was terrifying about him."

Christopher Nolan stands next to the Bat-Signal

Ledger's outstanding performance as the Joker garnered him a nomination and win for Best Supporting Actor at the 2009 Academy Awards. It was the first (and so far only) time a cast member of a superhero movie had won in an acting category at the Oscars. It's a performance that Hollywood has tried to replicate many times over, and it's something that Nolan believes was important for film history.

"I took huge pride in having been in any way involved with this great performer and his legacy. He was an extraordinary person and an extraordinary actor. And for him to be recognized in that way, I think, was very meaningful for his family and meaningful, I think, for film history. What he contributed, and he contributed in many different ways to film history, but that it be marked in that way, I was very proud to be a part of it."

The Dark Knight, and Christopher Nolan's Batman trilogy as a whole, is frequently considered the gold standard of superhero filmmaking. And even though the movie turns 10 years old this year, its impact on the industry hasn't waned, despite an influx of superhero movies having debuted since its release in 2008.

More: The Best Reviewed Movies Since 2000

Source: BBC Radio 1