A call from police in New York came four hours later.
He had trouble finding the time to grieve during those confusing first few hours.
"He was bulletproof. All young people are bulletproof."
At the time of his death, Heath had recently finished filming The Dark Knight and was working on The Imaginarium of Doctor Parnassus.
"It's difficult to trace and difficult to understand," Mr Ledger said.
"He was warned by his sister the night before but he still got caught out."
Getting rid of the stigma surrounding addiction-which was that the kind of people who suffered from it brought it on themselves and were not worth saving-was one of his goals in speaking at the conference, Mr Ledger said.
"We need to be compassionate with these people. It's a disease and you need to encourage these people to speak out."