Jake Gyllenhaal is opening up about the death of his friend, actor Heath Ledger, saying it still affects him deeply.

"Personally, it affected me in ways I can't necessarily put in words or even would want to talk about publicly," Gyllenhaal told People magazine in a recent interview.

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Ledger died of a prescription drug overdose.

Gyllenhaal and Ledger co-starred in the 2005 film, Brokeback Mountain. "In terms of professionally, I think I was at an age where mortality was not always clear to me,” Gyllenhaal revealed.

“I think it [gave me] the experience of, ‘This is fleeting.’ And none of the attention or synthesized love that comes from the success of a film really matters at all,” Gyllenhaal said.

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“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.”

Of making the film, he said. "We were super young.”

“We had no idea what the movie was going to become."

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