Jake Gyllenhaal's Grueling 'Nightcrawler' Transformation

Gyllenhaal explains how he prepared for this gritty new role, including spending time with a real news stringer.
7:25 | 10/28/14

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Transcript for Jake Gyllenhaal's Grueling 'Nightcrawler' Transformation
Every night in cities across America, there are people who do nothing but listen to police scanners and wait for word of murders and car crashes. And then they drive to the scene as fast as they can and they take video. Officially, these people are known as stringers. Unofficially, they've been called the paparazzi of pain. And they are now the subject of a major new movie starring Jake gyllenhaal. ABC's Byron Pitts takes us inside their world. Start filming. Reporter: Hollywood's latest inspiration. Those horrific high speed, heart-stopping, caught on tame moments. Will this be on television? Morning news. Reporter: In "Night crawler," Jake jillen hall plays the man behind the camera. I have some footage for sale. Reporter: A hungry flee lancer who will stop at nothing. You have to call the cops. And we will. At the right time. Reporter: To make it big. Who is Lou? He's part of a generation of people that are looking for jobs in a world where jobs are redefining themselves. He's a thief. Sort oaf a petty thief at the beginning of the movie. Runs across this accident scene and sees guys who film accidents and crimes and stuff for local news and he finds his calling in an instant. You have a good eye. I want you to contact me when you have something. Something like this? Think of our news cast as a screaming woman running down the street. Caught on camera, a high speed chase. Pursuit suspect is in custody this morning. Reporter: That may be harsh satire, but we've all seen it. The chases. The crashes. The chaos. The scene early this morning in the 400 block. Reporter: And the just plain unexpected that bring in viewers. The movie dives into the psyche of this twisted world. Why so dark, this character? There's a special type of personality that can do this job. You have to love the night. Reporter: Or hate the sun. Actually found myself really disappointed when the sun would come up as I was playing this character. Reporter: To become Lue, gyllenhaal drew inspiration from one of southern California's other nocturnal creatures. What did you do to prepare for this role? I wanted him to look like a coyote. In order to do that, I had to look hungry and be hungry. Reporter: He means that literally. Gyllenhaal lost nearly 30 pounds for the role. By eating a diet of kale salads and going on long runs. I would do this loop, this ten-mile loop and I would picture myself as a coyote. Like I was running with my brethren or something after awhile. Reporter: Gyllenhaal's research included heading out after dark with this man, Howard. I'm an adrenaline junky. Reporter: He's a real life Los Angeles night crawler, who for 15 years ths has been selling video. Gyllenhaal found him after the character was written, but they still got a dose of harsh reality. First thing he said, you guys want to wear the bullet proof vests or not? You got shot at. We were racing at 100 miles an hour. We arrived at a car accident where three young women had been ejected from a car. It was very horrible, bloody, violent. Howard, very professionally, got out his camera, filmed it, cut it together. What was the thinking there? I didn't ask that to hear myself speak. This transformation is very different from many of the roles I played in the past. It was a physical one, a mental one. But I brought me to a world that was different than any role I've ever played. Reporter: So, we hit the streets to see what his adrenaline-packed nights are really about. What's the goal? Goal is getting something like a crash on tape. We will go to minor crashes on the freeway and sometimes we're there before they block the freeway and we get cars piling into everything and that's worth a bit of money. Reporter: Stringers can make a few hundred to thousands of dollars a night. In the movie, Jake plays Lou. Are you Lou? I'm Lou to a certain extent. Without the felony aspect. Reporter: During downtime, he waits in a parking lot with easy access to the highway. Possible fatal crash. Multiple calls have come in. Three calls. Possible fatal crash. Come on. . If it's a good crash and it's visual, that would definitely sell. This could be a good start to the night. Reporter: It's a job where good is relevant. This is going to be a dump the car and run. Come on, come on, come on. Move! Reporter: As he arrives, police are already on the scene. Everybody clear out! This is not a safe area. That's a brutal impact. I mean, I've seen a lot of crashes, but that is brutal. I just spoke to the fire department. They said the guy was in full arrest, so, it's not looking good. Reporter: There's a certain stoicism about his sat tuesdaattitude. You've been called paparazzis of pain, video vultures. They work for you? No, no. Reporter: Give me what you prefer? I'm a stringer. Reporter: He also served as the technical adviser on the film, helping to translate the complicated language of the emergency scanners that contain hidden tips, if you know how to decipher femme. 25,000 channels. They have every fire department. Armed robbery. 187 is a homicide. Okay, we have an officer needs help. I don't know if it's an officer down. He was screaming for help. Lapd everywhere. I have never seen a policeman get so many resources into one area so quickly. I will never ask you to do anything that I wouldn't do myself. Reporter: In the movie's over the top plot line, gyllenhaal doesn't hesitate to violate all sorts of professional and legal codes. I think you withheld information. That would be a very unprofessional thing for me to do. It would be murder. Reporter: In this movie, your character crosses a line, more than a few times. I think Lou's opinion of that is he's an artist. My opinion is that I think all the things he says are right but I think the things he does inevitably do cross that line. Reporter: The character in the movie clearly crosses the line. Do you ever cross the line? No. Reporter: You hesitated. There's a few laws we break. We have to get to the scene quite fast. We do -- go over the speed limit sometimes. We are very, very safe, though. 911, what's your emergency? Reporter: Gyllenhaal found himself identifying more and more with all the different shades of his character. There were pieces of me that are in Lou. I would say a generation, I speak for myself, but I would say a generation has been raised with the success at any cost. You know, that ambition is admired. But at the same time, I think it can be very easily misused. We want all of it. Reporter: In "Night crawler," success at any cost will make your palm sweat. For "Nightline," I'm Byron Pitts in Los Angeles. And "Nightcrawler" is in theaters on Halloween.

This transcript has been automatically generated and may not be 100% accurate.

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