On a personal level, Ledger's 2008 death from a drug overdose "affected me in ways I can't necessarily put in words or even would want to talk about publicly," Gyllenhaal says. "In terms of professionally, I think I was at an age where mortality was not always clear to me."
Gyllenhaal was 27 when Ledger died, and was just 23 when he filmed the landmark movie that earned him and Ledger Oscar nominations.
"I think at the time, I assumed everyone would notice - and they did with Heath dying, but I think it [gave me] the experience of, 'this is fleeting,'" he says. "And none of the attention or synthesized love that comes from the success of a film really matters at all. What matters is the relationships you make when you make a film, and the people you learn from when you're preparing for a film. That changed a lot for me."
Watch the full interview here: