LOS ANGELES — When I spoke to "Inside Llewyn Davis" star Oscar Isaac a few weeks back, he had just come from seeing "Another Day, Another Time." The music documentary, which airs on Showtime Friday night, covers the Sept. 29 concert celebration of American folk music held at New York's Town Hall in honor of the Coen brothers' latest film.

At the time, Isaac noted how T Bone Burnett, the mind behind the film's soundtrack and the concert, seemed to hover over the proceedings like an ethereal presence. "I don't think he even really says anything but he's just floating around," the actor said. "I mean, literally, sometimes he just walks up behind someone that's playing and just stands there, feeds the mojo, and walks away."

Talking over a cappuccino at one of his favorite Brentwood haunts, the music maestro laughs at the quote. "There is a bit of that," he concedes in his smooth yet weathered tone. "I feel like I walk around until it sounds exactly right. Especially when a lot of musicians are playing, you walk around until you can sort of hear everybody. You become sort of a conduit."

The concert is populated by some of the same names who made a trip to Los Angeles for a west coast distilled version of the show last month, including Punch Brothers ("Chris [Thile] is the greatest musician out there today," Burnett says in no uncertain terms), Willie Watson, Rhiannon Giddens and The Milk Carton Kids, including appearances from Joan Baez, Jack White and Marcus Mumford. And there's a stand-out quote from Mumford in the film, in fact, that begins to get at the heart of what "Inside Llewyn Davis" — indeed, the whole of artistic expression — is all about.

"My temptation is to try and make everything big," the Mumford & Sons frontman says before taking the stage for a haunting solo ditty. "So going back to the small and the quiet is quite exposing. It's terrifying. It's good, though, to do things that are scary."

Says Burnett, "Exposed, that's the beautiful thing about folk music. It's sung by people without affectation. They're not experts."

The show and resulting film were turned around very quickly, and Burnett says there are even plans to cut each performance so the songs can be released as video singles. A three-disc vinyl album from the show is also planned, and early considerations are being made for a full-blown tour next fall to help further catapult the careers of these musicians. "I don't want to withhold anything from it, really," he says. "And I want to be able to give it to the artists. Some of the artists whose pieces didn't end up being in this film, they can put them online or sell them as singles."

But much of the material is interesting in that it doesn't necessarily feel like the music of the era depicted in "Inside Llewyn Davis." That early Dylan folk era represented by a certain sort of pre-rock aesthetic gives way to more of a bluegrass bent, not too far off from the work that took flight in the wake of "O Brother, Where Art Thou?," another Burnett/Coen collaboration.

"It's more filled out," Burnett says of the featured tracks. "And even in ['Inside llewyn Davis'], Oscar's tone doesn't have anything to do with folk music at all. Other than Jeff Buckley — if Jeff Buckley was a folk singer — but that was the 80s. He sounds like that, or Ryan Adams. He's in this much later wave of singers. And we didn't do it with 'O Brother, Where Art Thou?' either. We don't go for the form. We go for the content. We get the content right and let the form be whatever it wants in the room when you're doing it, rather than trying to force a form over something.

"The other thing is you can play these games because no one's heard these songs anyway. They think we wrote them all. Same thing that happened to Dylan. Everybody just thought he wrote all those songs!"

That's the fascinating thing about this music and part of what makes it the beating heart of the Coens' film. "If it was never new and it never gets old, it's a folk song," Davis says at the beginning of the film.

So much material was filmed at the New York event that Burnett says there may yet be another movie at some point. But for now, "Another Day, Another Time" represents a well-remembered evening dedicated to his life-long love: the cultural anthropology of folk music, offered up by some of the greatest unsung musicians of our time.

"That's it," he says. "This is completely what my life's about at this point, at 65. It's totally about exactly that. Who are we? How are we going to live? How are we going to treat people?"

"Inside Llewyn Davis" is currently playing in limited release. It will expand wider on Dec. 20. We'll have more from Burnett on his contribution to the film in the coming weeks.

"Another Day, Another Time: Celebrating the Music of 'Inside Llewyn Davis" airs on Showtime Friday, Dec. 13 at 10pm ET/PT.