He had apparently been suffering from depression, not sleeping for weeks and weeks. That, plus the pneumonia, undoubtedly rendered him extra-susceptible to a bad reaction to whatever drugs he took. A dose that would not normally be an overdose might well have killed him.

His recent portrayal of The Joker, in the yet-to-be-released next Batman movie, is said to be extremely "dark," much different from Jack Nicholson's take on the same character a few years ago. Ledger makes The Joker into an evil psychotic serial killer with no comic-book overtones; apparently, he immerses himself very deeply into all his roles (the generally excellent results would indicate as much), and this role may have taken him into depths he was unable to escape.

The Today show showed a few brief clips of Heath as The Joker, and he certainly did look demented and scary