While he played more than one part in his short career, Heath Ledger will forever be remembered for his portrayal of The Joker in The Dark Knight. The film was both a critical and commercial success, with the majority of plaudits focussing on Ledger's manic and dark character - which launched him to a new level of superstardom. However, this all happened posthumously, as Ledger died in January 2008, 10 years ago today and 6 months before the film was released.

A few months before his untimely death, the actor had opened up about the effects that the role of playing that Joker had on him both physically and mentally. Ledger claimed that he was constantly tired due to a lack of sleep as his mind was constantly thinking.

He had just wrapped up filming for the Bob Dylan biopic, I'm Not There, and was medicating his insomnia with various sedatives. Speaking about the difficulties he was having at the time of filming, Ledger told The Times:

“Last week I probably slept an average of two hours a night. I couldn’t stop thinking. My body was exhausted and my mind was still going.”

The actor's death was ruled as accidental at the time, with the toxicology report showing traces of oxycodone, hydrocodone, diazepam, temazepam, alprazolam and doxylamine in his system.

Of course, due to the popularity and status of Heath Ledger, there have been various documentaries made about him and the final months before his death. One of the most recognised of these was 'Too Young To Die' which looked at the lives of celebrities who had died at a young age. In the episode regarding Ledger, the German filmmakers came across the diary that he had made when creating the character of The Joker.

The diary was shared by Heath's dad, Kim, and reveals the extent of the actor's commitment to the character and includes manic musings and images to help Heath understand the disposition of The Joker. The diary also contained several of the inspirations behind the character, including clowns, hyenas, comic strips and Alex from A Clockwork Orange.

Kim can be seen flicking through the diary while confirming that his son put himself into voluntary exile in order to enter the mindset of The Joker.

"He pretty well locked himself up in a hotel room for weeks. He galvanised the upcoming character. That was typical of Heath. He would do that. He liked to dive into his characters, but this time he really took it up a notch," Kim said in the documentary.

Chillingly, one of the pages of the diary has the words "BYE BYE" written across it in large letters.

The crew behind The Dark Knight were in the process of editing the film when Ledger died. Speaking about the tragic death, director Christopher Nolan said:

"It was tremendously emotional, right when he passed, having to go back in and look at him every day."

"But the truth is, I feel very lucky to have something productive to do, to have a performance that he was very, very proud of, and that he had entrusted to me to finish."

While plenty of people say that the psyche of The Joker was partly to blame for the death of the actor, co-star Christian Bale disputes this claim, recalling Heath saying: "[The Joker was] the most fun I've ever had, or probably ever will have, playing a character."

Whether The Joker did or didn't play a part in the untimely death of Heath Ledger, the world lost an incredible talent on 22 January 2008.