Transcript for Bradley Cooper Describes Taking on 'American Sniper' Role
Okay, here's the thing. It takes a lot more than fine acting chops to transform yourself from the American hustler to the American sniper. For Bradley cooper, it involved massive weight gain, rig use accent training and some real life hero worship. Tonight, he opens up to my "Nightline" co-anchor Byron Pitts. Reporter: In this heart-pu heart-pumping, highly patriotic film -- Don't pick it up. Reporter: Bradley cooper -- Drop it. Reporter: Does more than portray an American war hero -- Are you chief -- Yes, sir. Reporter: He humanizes it. You saved my life. You did? Yes, sir. Reporter: Chris Kyle, the most lethal sniper in U.S. Military history. 150 confirmed kills. The enemy feared him. His peers on the battlefield revered him. All these guys? They know your name. They feel invincible with you up there. They're not. They are if they think they are. Reporter: Based on the "New York times" best ell seller, "American sniper" chronicles Chris Kyle's success and repercussions on his psyche and his personal life. I'm stateside. You're home? I guess I just needed a minute. The kids are dying to see you. It's been nine months. I'm coming home. Reporter: There's early buzz of a third possible Oscar nomination. He was previously only noted for "American hustle" -- What is there to confirm? Reporter: And "Silver linings playbook." Your poor social skills. You scare people. I tell the truth. You're mean. Reporter: "American sniper" bears the intimate details Clint Eastwood is known for. Clint was right there for that moment. Reporter: Next to you? When I was on that gun, he was right there. Sort of talking and that's the way it was shooting the whole movie. Clint was there all the time. Reporter: For all the intensity on screen, cooper and east the wood shared moments off it. Like the director wondering if his star should actually lift the 400 pounds you see in this scene eight times. He just said, do you really have to lift that weight? I said, yeah, yeah, it will be fine. Don't pull anything. Reporter: Cooper gained 50 pounds for "American sniper" and trained with a dialect coach to get that Texas twang. You said thing with thin. You wouldn't do Saturday, Sunday, you know, it's very interesting. It's almost like learning a foreign language. Reporter: You stayed in character the whole time? Even, like, talking to your girlfriend? Yeah, god bless her. Yes, yes. Reporter: Cooper's girlfriend is 23-year-old british model and actress suki Waterhouse. Though he is private about it, their relationship, much like everything in cooper's life, has been relentlessly chronicled by the paparazzi and his career by the press. He's the shirtless cover star of "W" magazine's movie issue. These days, cooper's returned to the theater, starring in "The elephant man" on Broadway. Performing to packed audiences. So, how does one of the most celebrated actors get to work every day? The subway, of course. That's crazy. Is it? I mean, it doesn't feel crazy. I always take the subway here. Reporter: How is it different being paragraphed by the paparazzi -- By the iPhone? I have a lot of maneuvers. If you sense somebody, you know, it's a constant thing that you do. People are very covert, man. They do a lot of this thing. You see it hanging out there. You know? There is also this, Reading the thing. You don't -- you can read it there. Reporter: For all his success, there's a down to Earth guy next door quality to Bradley cooper, and he gives full credit to his family. He dotes on his late father, Charlie cooper. My dad was my man. Reporter: What do you think he would have said about this performance? Growing up, I remember thinking, when I was a kid, the idea of him crying, never. I started to see him really sort of dip into the emotion of his life. Reporter: His family enabled him to flourish. I never, ever grew up with parents that pushed me to do anything other than something that was going to make me happy. I know that's a rare thing. I was lucky, because I had such a self-generated engine in myself that I never needed them to push me. Reporter: Cooper graduated with honors from Georgetown. Studied acting at New York's the new school. Appearing on its program "Inside the actor's studio" with guest Sean Penn. My name's Bradley cooper. I'm a second year actor. What was it like to revisit a character, Eddie, after a ten-year hiatus? Reporter: Success seemed far from certain. You want to go for a ride? Reporter: From a one-time role on "Sex and the city" -- I have to go home. No way. Reporter: To his recurring one on "Alias," bit parts in movies. But it all changed six years ago. To a night the four of us will never forget. Reporter: With "The hangover." How does one go from "The hangover" movies to the very serious, in part, dark characters in some ways? Well, I don't see any difference at all. Reporter: Huh? I don't. Phil to me was a real character that wasn't me. Tracy, it's Phil. Where the hell are you guys? And Todd Phillips and Zach and ed are as good as Christian bale and Jeremy Renner. Those guys are serious gangster actors, I mean, they are no joke and you have to bring it hard every time you're with them. So, I see it as the same thing. You want to die? Is that what it is? No. Then just tell me. Why do you do it? I want to understand. Babe, I do it for you. I do it to protect you. No, you don't. Yes, I do. I'm here. Your family is here. Your children have no father. I have to serve my country. You don't know when to quit. Reporter: There is very little humor in "American sniper." It delves deeply into the high cost of war. The aftermath of battle when the adrenaline rush has passed. Keeper only spoke to Chris Kyle once by phone. He wanted to tie me up, put me in the back of his pickup truck and ride around. I found that as a compliment. Reporter: A compliment? Pretty? Thanks. 38, pretty? Reporter: But before the movie went into production, life took a tragic Hollywood turn. Shocking killing of Chris Kyle. The most lethal sniper in U.S. History was gunned down at a Texas shooting range over the weekend. The man arrested, a troubled former marine whom Kyle was trying to help. Reporter: After his death, cooper met with Kyle's family, including his father, Wayne. I tried to get to know him. I wanted him to get to know me. I said, we're going to do right by your son. And I meant it. And I think he saw that. You got eyes on this? Reporter: Whether he wins any major awards or not, Bradley cooper has already achieved two major successes. Honor an American hero and his farthest dream for his son to live a full life. If he was still alive, he'd be 74 now. Oh, he would be a mess. I think every time he would see me, he would cry. Reporter: For "Nightline," I'm Byron Pitts in New York. Nice. "American sniper" will be in theaters nationwide January 16th.
This transcript has been automatically generated and may not be 100% accurate.