Metallica reveals how they decided on Lady Gaga duet for Grammy Awards 2017: ‘She said ‘yes,’ took about 12 seconds’

— -- Metallica and Lady Gaga’s joint performance was one of the most talked about moments at the Grammys, and even despite a technical mishap, they still lit up the stage – quite literally, with lots of pyrotechnics.

But how did a heavy metal band decide to team up with one of the world’s biggest pop stars?

According to Lars Ulrich, Metallica’s drummer and founding member, it was as simple as just asking her.

“Gaga has been a long self-professed metal head, a life-long Metallica fan,” Ulrich told “Nightline.” “We have a mutual friend who doesn’t live too far from here and we were at dinner six weeks ago, four weeks ago. And I said, ‘We’re doing the Grammys, would you like to join?’ And she said, ‘Yes.’ It took about 12 seconds. And that was it.”

“Nightline” was granted exclusive access to a pre-show practice with the band for the Grammys in Los Angeles on Friday night. Watch the full story HERE.

Metallica guitarist Kirk Hammett said Gaga quickly picked up on what they envisioned for the fiery Grammy performance.

“By the time she [spent] what, 35 maybe 40 minutes with us, she totally got into the song, worked out an arrangement with James [Hetfield, lead vocals and guitar], totally got a feel for the song.”

“Somebody that can actually sing, like Lady Gaga… It was pretty seamless,” added bassist Robert Trujillo.

Metallica is a group that lives for live performances, Hammett said, that’s the best way to get a feel for the band’s “message.”

“It’s very honest what we do. We strap on our instruments and what you see is what you get,” he added.

“There’s like a groove factor and what you’re feeling is the pulse of that,” Trujillo continued. “And that’s what moves you.”

Trujillo is famous for his “crab walk” on stage during performances, which he said was inspired by Ozzie Osbourne from when Trujillo used to play with Black Sabbath.

“[Osbourne] would come up to different band members, in this case it was me, and he’d get in my face, sometimes he’d pull your hair or put his arm around you really hard, he’s a rough cat on stage,” he said, laughing. “I loved it, I loved what he was doing. So then I started getting lower and lower.”

Ulrich started Metallica when he was just 17 years old. He and Hetfield have been bandmates for 35 years.

“It’s the longest relationship I’ve had with anybody basically other than my father,” Ulrich said.

Although the bandmates are now all in the 50s, they are still rocking hard and working on new music. In just a few months, Metallica has sold nearly 1 million copies of their latest album, “Hardwired…To Self Destruct,” but self-destruct is far from what they have done. The band has sold over 100 million albums, has played massive tours on every continent – even Antarctica in 2013 – and won eight Grammys.

All of this might never have happened. Ulrich came to the U.S. in 1979 from his native Denmark to become a tennis player.

“I’m sort of the black sheep of the tennis side of the family because my father was a professional tennis player,” he said. “His brother, my uncle, was also the top tennis player in Denmark. My cousin, you know, has won Danish championships… Music had always been my passion and my hobby, waiting in the wings to take over and it did.”

Hetfield became involved after he simply answered an advertisement for a guitar player.

“My mom used to take care of my dad, sort of facilitate all the shenanigans all over the world – the travel, the business, this and that, and so I think I inherited that,” Ulrich said. “So when James and I started I was the mom -- I was my mom! I’ve never said that before!”

A 2004 documentary called “Some Kind of Monster” showed a band with some internal issues, which Ulrich acknowledged.

“It’s a bunch of guys who have never really had an in-depth conversation about how they’re feeling and what’s going and what they think about everything for 25 years and all of a sudden it happens and all hell breaks loose,” Ulrich said.

These days, the band said they have slowed down some. Hammett said they won’t tour for more than two weeks at a time, and while the band used to be the primary thing in all of their lives, Ulrich said they are now focused on family. Ulrich himself has three kids.

“We take care of our kids, take care of our families, our wives, take care of all our domestic responsibilities and then we get a chance to run away with a rock and roll band and that’s a lot of fun,” he said. “And to be able to do that at 53 is pretty cool.”

But they are still playing. In fact, Metallica just announced a new U.S. stadium tour starting in mid-May.

“It’s pretty crazy that people still, 35, 36 years into a career still come, still care, still are passionate. And there’s still a turnover of newer, younger fans,” Ulrich said. “Hand on heart, we’re somewhat good at what we do, our little niche.”

ABC News' Nick Watt contributed to this report