Trent Reznor, Nine Inch Nails frontman and music industry polymath, calls his gig as a go-to composer for Hollywood film scores an “accidental career.”

Known for two decades as a transgressive industrial rocker prone to loud, punishing darkness, Reznor, with his longtime collaborator Atticus Ross, fell into a subtler role beginning with David Fincher’s “The Social Network” in 2010. Their score won an Academy Award.

Since then, they have balanced composing — their recent work can be heard in the climate-change documentary “Before the Flood” and the film “Patriots Day,” set during the bombing of the Boston Marathon — with writing new Nine Inch Nails material (an EP, “Not the Actual Events,” came out last month) and Reznor’s job as a hands-on executive at Apple Music.

Yet when long-form filmmaker Ken Burns (“The Civil War,” “Baseball”) and his co-director, Lynn Novick, approached Reznor’s manager more than three years ago about an even more ambitious project — scoring his 10-part, 18-hour documentary about the Vietnam War — the musicians gamely added it to their pile of disparate responsibilities.

“I don’t think he finished the sentence before I said, ‘Yeah, we’re in,’” Reznor recalled in an interview. “That’s a no-brainer.”

Though he admitted the scale of the project was “a bit daunting” compared with your average movie, that’s also what made it worth tackling. “Not every opportunity to score a film feels like it’s an artistic endeavor,” Reznor said. “Rather than just accumulating projects, we’ve tried to select things that challenge us and feel exciting.”

“The Vietnam War,” which premieres in September on PBS, will ultimately feature more than two cumulative hours of original music from Reznor and Ross, along with reworked bits from Nine Inch Nails songs and their scores for “The Social Network” and “The Girl With the Dragon Tattoo.” Yo-Yo Ma and the Silk Road Ensemble also contributed music to the documentary, while era-defining hits from the Beatles, the Rolling Stones, Bob Dylan and more will be used as well.

For Reznor and Ross, the work was slow and deliberate, beginning not with scenes from the film but a blueprint of moods. The composers requested “a laundry list of emotional settings that we could draw upon” from Burns and Novick, Reznor said. “A lot of it was about tension and fear.”

The filmmakers also provided field recordings from the period to inspire the compositions. “Mortars, helicopters, people talking — we would process those sounds and use them as rhythmic starting points,” Ross said.

Working simultaneously on several projects provided the musicians a sense of creative freedom within each, they said, though gloom remained a constant theme.

“The tone of concern in ‘Before the Flood’ led perfectly into the pure dread, evil and melancholy that we explored in ‘Patriots Day,’ which also provided us the momentum to make the Nine Inch Nails record ugly and confrontational,” Reznor explained. “We could go deeper in each pool knowing it wasn’t forever. We could take a break from the end of the world by going into a terrorist bombing, or take a break from that by making an ‘I want to kill myself’ album.”

This year may be somewhat calmer. Reznor said that with no additional feature film work scheduled, he would prioritize new Nine Inch Nails music in 2017. (He and his wife also had a fourth child last month.) And while he called a full tour unlikely, the band will headline the Panorama music festival in New York on July 30, its first live show in nearly three years.

More opportunities could arise, and Reznor and Ross have grown more confident in their balancing act — “as long as we don’t put off writing the paper until the night before,” Reznor said. “We’re getting better at that.”