Bill Malone still remembers well his days as a graduate student at the University of Texas in the 1960s when the seeds for his definitive book “Country Music, U.S.A.” were planted. It all began innocently enough for the East Texas native, who used to sing country tunes with friends at Threadgill’s on North Lamar Boulevard.

“Well, it was kind of an accident,” he says of the book's origins while he was getting a Ph.D. in history under department chairman Joe Frantz. “I sang all the time around the department. I was obnoxious, you know, always humming and singing. After I finished my prelims for the doctorate, I took a trip to Houston with Dr. Frantz and some other graduate students, and we sang all the way down. I think we were going to the Bluebonnet Bowl.

“At some point, he said, 'You like this music so much, why don’t you write something on it? Why don’t you write a history of Nashville publishing?' Because Nashville was really reviving at that time from the doldrums it had been in during the rock 'n' roll period.

“I said, ‘You can do that? You can actually write about something you love?’ Because nobody I knew in graduate school at that time was doing that. They were writing on assigned topics, sort of grueling through it. So I wrote the dissertation, and then the dissertation was made into a book in 1968. It’s now had 50 years of life.”

Little did Malone realize back then that “Country Music, U.S.A.” would someday become a vital source document for a 16-hour series from preeminent PBS documentary filmmaker Ken Burns. “Country Music,” which will air in two-hour segments Sept. 15-18 and Sept. 22-25, follows the 2001 series “Jazz” as the second musical-genre project for Burns, whose films on the Vietnam War, America’s national park system and other subjects have made him a household name ever since 1990's epic “The Civil War.”

“His book was the bible for us,” said Dayton Duncan, who wrote the script for “Country Music.” Duncan, Malone and the project’s producer, Julie Dunfey, visited Austin last week for a free sneak-preview screening of clips from the series at ACL Live. In an hourlong interview before the event, they discussed how “Country Music” was made over an eight-year span that began while Burns, Dunfey and Duncan were wrapping up the 2012 series “The Dust Bowl.”

“His book was where I started,” Duncan said of “Country Music, U.S.A.,” which last year was republished by University of Texas Press in a 50th-anniversary edition with newly added content. “Our approach in making a film is more biography-based and more storytelling, and part of that is understanding that it can’t be as exhaustive and detailed as his (book). But you needed to know that (material) before you could start zeroing in on the stories that you did want to tell. So it was essential to us.”

In “Country Music: An Illustrated History,” a 560-page hardcover volume issued by Knopf this week in conjunction with the series debut, Duncan and Burns gave a special acknowledgment to Malone’s book. “We make note of the fact that when Bill started out on ‘Country Music, U.S.A.,’ the notion that an academic would treat this topic seriously was sort of scoffed at," Duncan said. "And his book now, 50 years later, is the headwaters.”

DUNCAN KNOWS a bit about headwaters. He had a primary role in the 1997 Burns film “Lewis & Clark: The Journey of the Corps of Discovery,” which traced the 1804 path of the two American explorers up the Mississippi and Missouri rivers in their search for a northwest passage to the Pacific Ocean. Duncan had written a book on Lewis and Clark in the 1980s, and he was one of the primary on-camera storytellers for that film. But working on “Country Music” was a very different experience.

“My relationship with country music would make a great country song,” he joked in a panel discussion that followed last week’s preview screening at ACL Live. In our interview, he elaborated on that: “You know, we were in love once and then we fell out of love, and then we reconciled, and then we’ve had our fights, but the love is there."

But the full-on immersion in the subject that was necessary to make the PBS series required a great deal of research. “When I started this, I was not anything approaching an expert on country music,” he said. “What I might have thought I knew about country music was just a drop in the big ocean.”

Dunfey's history with Burns goes back to the “The Civil War” and a much shorter 1988 film about American artist Thomas Hart Benton, whose mural “The Sources of Country Music” figures prominently in the opening episode of the new series. She says the learning curve was similarly steep for her. In college at Dartmouth in the 1970s, she’d been a fan of Willie Nelson, Waylon Jennings and the Nitty Gritty Dirt Band, “but beyond that, not a whole lot” of country music had been part of her life, she said.

“So for me it was a real discovery. We all approach the film as a process of discovery; we don’t approach it as experts," she said. "But I took a deep dive, and any time we went to interview anybody, I would listen to their music over and over and over again and read all the reports and books that we had assembled. And now — I’m quoting Emmylou Harris from our film — I became obnoxious in the way only a convert can be obnoxious. And (country music) is what I listen to."

Those interviews for the film, which Dunfey arranged and most often were conducted by Duncan and/or Burns, ultimately totaled 175 hours of footage with 101 different sources. They range from living legends Dolly Parton and Willie Nelson to recently departed greats such as Merle Haggard and producer Harold Bradley to younger-generation luminaries including Rhiannon Giddens and Old Crow Medicine Show’s Ketch Secor.

What there’s not much of in the film, with the exception of Malone, is on-camera interviews with academic figures. That’s largely because many of the genre’s key musicians are vastly knowledgeable about the genre’s past and present, Duncan and Dunfey said. Thus a great deal of background comes from the likes of country stars Marty Stuart and Rosanne Cash, or Austin’s own Ray Benson, the Asleep at the Wheel frontman who also took part in last week’s panel discussion at ACL Live.

READ MORE: Our 2018 interview with Ray Benson

That helps differentiate “Country Music” from typical Burns projects. “In a classic Ken Burns/Florentine Films film — not just ones that we (Duncan and Dunfey) have worked on, but ones other people produced and wrote — generally the people who appear on the screen are historians or experts on that topic,” Duncan said. This one involved much more input from performers, producers and songwriters. Duncan offers Benson as a prime example. “He’s a musicologist, whether you want to call him that or not. He had a lot of knowledge about the music, but coming through the filter of an artist himself.”

BENSON HELPED the producers weave considerable content about key Texas figures into the 16-hour series. Nashville was an obvious focal point, and the filmmakers understood that country music, by its very name, implies a broad geographical reach that covers all of America. But they found plenty of Texans to feature in the final cut. The sneak-preview screening included passages on Nelson, Bob Wills, Kris Kristofferson, Guy Clark and Townes Van Zandt; Duncan joked that if they’d shown all the Texas-related excerpts available, the crowd would have had to stay till midnight.

Not all of the primary Texan subjects lived long enough to be interviewed for the series. Duncan brings up George Jones and Ray Price as two legends who “had agreed to be interviewed, and we were about a month out from when we were going to be doing them, and there was radio silence from their people. We knew that that probably was not good news.” Both Jones and Price died in 2013.

READ MORE: Ray Price changed country music while staying true to his own sound

Twenty of their 101 interview subjects have since died, which makes the viewing of “Country Music” a sometimes bittersweet but even more valuable experience. The producers got great insight from Haggard, who Duncan remembers as being a fountain of information about other artists as well as his own songs and career.

“When I sat down with Merle Haggard about two years before we interviewed him,” Duncan recalled, “he said, ‘What do you know about Emmett Miller?’ And I said, ‘Well, I don’t know much about Emmett Miller.’ And he kind of looked at me and said, ‘You do know who Jimmie Rodgers is, right?’ And I said, ‘Yes, sir, I do know that.’ He said, ‘Maddox Brothers & Rose?’ I said we don’t know a thing about them. And then I said, ‘I’m writing it down.’ He says, ‘Well, good, because they get overlooked, and they’re very important.’ So there was that kind of stuff.”

In pairing the interview material with performance footage, an invaluable source was the archives of “Austin City Limits,” Duncan and Dunfey said. The program’s executive producer, Terry Lickona, moderated last week’s panel discussion at ACL Live, where the TV show’s iconic skyline backdrop loomed behind them as they spoke.

RELATED: 'Austin City Limits': a rewarding history, a challenging future

“When we were looking for performances, if we were talking about a song or an artist, we often found it” in the ACL archives, Dunfey said. And, Duncan added, “the production quality was so good.”

THERE WAS NO “Austin City Limits” back in 1939, when a 5-year-old Bill Malone first got hooked on what was then commonly referred to as hillbilly music after his father bought a Philco radio for the family. “It’s a love affair that began that long ago and still continues today,” Malone said.

Benson, whose band kicked off the inaugural season of “Austin City Limits” after Willie Nelson did the pilot episode, remains grateful for Malone’s efforts. A Philadelphia native, Benson moved here in the early 1970s and says that when he started his career, “one of the first things I did was read Bill’s book.”

To see that book play an integral role in the “Country Music” series is a big moment for Malone, he confessed to the sneak-preview crowd at ACL Live last week. “It’s a validation of the culture I grew up in,” he said, “and the music I devoted my whole life to.”