From "Brokeback Mountain" to "Jarhead," Jake Gyllenhaal is a chameleon, adapting to a wide range of roles -- but this is the first time a movie has had a major impact on his soul.

“Dave [David Ayer, director] told me right off that this was going to affect my soul. 'Friends are going to say, That’s the Jake I’ve always known, but somewhere deep inside, you’ll know otherwise,'" Gyllenhaal told Details magazine of his forthcoming project, "End of Watch" in which he plays a South Central LA cop and even witnessed a real gang shooting while researching his role.

The film required an extreme amount of mental and physical preparation, including fight training, shooting practice ("shooting past each other's heads, with live ammo") and dozens of police ride-alongs.

"Every journey starts with fear," Gyllenhaal explained. "And I could say that’s what I want to embrace now. A real experience. And I want, overall, to trust what I know is right. There have been many times when I haven’t. It’s what I’m asking myself: Where is the line? What is the line? There’s so much context, it can be almost impossible to find.”

But Gyllenhaal's next film, "An Enemy" is affording him some time off from his grueling "End of Watch" schedule.

“I don’t run anymore," Gyllenhaal said. "Do I take care of my body and take conditioning seriously? Yes. But exercising regularly doesn’t fit the energy of the character I’m playing now.”

"End of Watch" hits theaters September 21.

For more, pick up the September issue of Details.

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