However his sister Kate has spoken out about this misconception, saying that it couldn't be further from the truth.
At a Q&A panel at the Tribeca Film Festival, she said that she was shocked by the rumours.
"I was really shocked because that was him having fun," she said. "Every report was coming out that he was depressed and that [the role] was taking this toll on him, and we're going, honestly, it was the absolute opposite. It couldn't be more wrong.
"He had an amazing sense of humour, and I guess maybe only his family and friends knew that, but he was having fun. He wasn't depressed about the Joker!”
Nearly 10 years after his death, a documentary celebrating his life and exploring his identity through his friends and family will be released for a short time only in cinemas.
I Am Heath Ledger will premiere in Heath Ledger's home town of Perth on May 9. It will show in Australian cinemas for one week only with limited screenings.
Watch the trailer:
This article first published on Marie Claire.