Transcript for Metallica reveals how Grammy Awards duet with Lady Gaga happened

So if you've had the privilege of hearing their dead growls in concert, you might be surprised to learn the members of metal car level-headed family men offstage. They've dominated the heavy melt scene over three decades. One of their keys to longevity, they say, is finding work/life balance. Downright zen heavy metal. Here's ABC's Nick watt. ??? Reporter: They even shared a Mike. After a tech mishap, gaga and metallica rocking the grammys. How did this happen? Gaga has been a sort of self-professed metalhead, life-long metallica fan. We were at dinner six weeks ago. And I said, we're doing the grammys, would you like to join? She said yes. It took about 12 seconds. Reporter: We were granted exclusive access to a preshow practice in L.A. Friday night. ??? you guys and gaga. Didn't immediately strike me as a natural marriage. By the time she spent, what, like 30, 45 minutes with us, she totally got into the song, worked out vocal arrangement with James, got the feel of the song. It was pretty seamless. ??? My poker face ??? Reporter: Gaga I'm familiar with, we know "Poker face" among others, a worldwide number one. She played the super bowl halftime show. ??? but these guys? Yeah, I've heard "Nothing else matters." ??? And nothing else matters ??? ??? never cared what they do ??? I'm going to come clean. I'm more of a Fleetwood Mac kind of guy. I finally understood what your kind of music is about because you can feel it. And -- Without a doubt. It was kind of eye-opening for me. You kind of enjoyed it. I did enjoy it. See, that's the thing. Our message comes by or comes through most accurately, most precisely, when you see us in a live situation. Reporter: We all know who metallica are. I mean, you can buy a t-shirt at H and M. As for "Sand man" -- ??? possibly the biggest metallica number. Now I need to know more. I started this band when I was 17. James and I have been together for 35 years. It's the longest relationship I've had with anybody, basically, other than my father. Reporter: James hatchfield leads vocals and guitar. 100 million-plus album sales later, after massive tours on every continent, eight grammys, they're still here, bigger than ever. Today they announced a north American stadium tour. Now we sit here in our early 50s and go, wow. This is really cool. They keep coming. Reporter: In just a few months they've sold nearly 1 million copies of "Hardwired to self-destruct" and over 36 years have, well, not self-destructed. You guys are still playing the grammys. You guys are still selling out stadiums. You're still number one. It's very honest, what we do. We strap on our instruments, the four of us, what you see is what you get. ??? Reporter: This all might never have happened. He came to the U.S. In 1979 from his native Denmark to become a tennis player. The black sheep of the tennis side of the family because my father was a professional tennis player. Music had always been my passion, my hobby, waiting in the wings to take over, and it did. Reporter: James Hatfield answered a want ad for other metal musicians to jam with. When James and I started, I was my mom. I never said that before. That's an exclusive on "Nightline." You never want to play stiff. Sometimes we look stiff. ??? I love the facial expressions, nice. So you look like you're pissed off, you look stiff. But actually, you're just bringing more impact to the strings. That's the way I look at it. Reporter: Bassist rob Trujillo has become famous for his crab pose on stage. I'm a 43-year-old man in tight jeans, I'm not sure this is going to happen. You always got to make sure your pants are up. Pull your pants up. Then you kind of have to, you know -- ??? I can't. Reporter: It's really about more than just the music. I love playing drums. But I love playing drums in a collective. Drumming's the gateway to the adventure. I'm not one of these guys that sits around, plays drums for eight hours, always has to tap on something. Reporter: 2004, "Fly on the wall" documentary "Some kind of monster" showed a band with some issues. I was straight up with you, I told you, . What have you been doing? A bunch of guys who have never really had an in-depth conversation about how they're feeling, what's going on, what they think of everything around them over 25 years. All of a sudden it happens. All hell breaks loose. Reporter: These days they slow down to smell the coffee. We don't tour for more than two weeks. We have a very strict two-week rule. We take care of our kids, take care of our families, wives, domestic responsibilities. Then we get a chance to run away with a rock 'N' roll band. Reporter: Coming to a stadium near you again this summer. It's pretty crazy that people still, 35, 36 years into a career, come, still care, still are passionate, and that there's still a turnover for newer, younger fans. Why do you think people still care and support you? Hard to answer that without -- I sometimes get a little uncomfortable, sort of hand on heart. We're somewhat good at what we do. ??? Reporter: Near does Reporter: Niche doesn't describe duetting with gaga at the grammys, but hey. Nick watt for "Nightline" in los Angeles.

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