I agree with Ian. I've read that his role as the Joker in the next Batman movie was very demanding. He's a method actor and I assume "becoming the Joker" can take a toll on one's mental health. In other news, Best Buy is cashing in on Heath's death only hours after his passing. They're informing their customers to "Remember a great actor through his great performances".
Would have to agree with this. I believe when Danny Devito did the Penguin he cut himself off from his family for 6 weeks because of the character he'd have to get into the mindset for.