Speaking as the movie received its big Hollywood premiere at the AFI Festival, the director admitted the decision to carry on hadn't come easily.
"It took several days, it was devastating," he said.
The director continued: "It took quite a while to believe that there was a solution to continuing the movie, I didn't even want to finish it really when Heath first died.
"But everybody around was so determined that they weren't going to let me get away with that and we were going to finish the movie."
Terry explained: "I wanted to use every bit of Heath that I could, and I was determined not to throw anything away."
According to the director, Heath's presence was felt on set even after his passing.
"Colin Farrell said he just thought that Heath was somehow there. There's no question Heath's spirit was massive, it just hung over all of us and that's why I only wanted friends of Heath to come and take over the part, because they knew him it was much easier for them to get into what he would have been doing.
"When people see the film everyone says the same thing, they can't believe it wasn't intended to be like this, that's what's so spooky."