Eric Roberts who plays Salvatore Maroni in "The Dark Knight", wrote a very nice tribute to Heath Ledger. Here is a sample:
"I did not see any indication of his having any problems of any kind. I have worked with other actors whose behavior is symptomatic of either depression or self-medicating, and I do believe in taking well-advised steps to reach out and help someone. But I saw none of that with this man and am in no position at all to comment. I used to have a terrible problem with myself resulting in problems with drugs, including the drug of alcohol. I wish I hadn’t been so good at scaring people into not approaching me. I needed their help. Though, truth is, several people tried and did a really excellent job, which I undid. Until one day…"
You can read the entire tribute here