Filmmakers Michael Bonfiglio, left, and Judd Apatow, right, flank Seth and Scott Avett at the SXSW world premiere of “May It Last: A Portrait of the Avett Brothers” on Wednesday, March 15, at the Paramount Theatre. Suzanne Cordeiro/for American-Statesman

Watching the world premiere of “May It Last: A Portrait of the Avett Brothers” on Wednesday night at the Paramount Theatre was kind of a meta experience. It’s not a concert film, but there’s plenty of musical footage, and at times the band would finish a song to the raptured applause that was captured on film — which was then followed by the raptured applause of the fans who filled up the Paramount. Is there an echo in here?

That’s part of what made attending the premiere special, of course. The audience consisted largely of die-hard Avetts lovers, many of whom had traveled long distances for this special event (judging from the locales name-checked in a Q&A session after the event). Some of them were probably here anyway for South by Southwest, but the North Carolina band’s most fervent followers are known for their devotion to the brothers, and this crowd undoubtedly was at least partly here for the film (along with the band’s subsequent 11 p.m. show at ACL Live).

PHOTOS: May It Last: A Portrait of the Avett Brothers red carpet at SXSW on March 15

Renowned directors Judd Apatow and Michael Bonfiglio, who took a left turn from their usual comedy avenues to do this movie, gave a brief introduction before giving way to the nearly two-hour screening. It’s a beautifully shot film, cutting concert footage in with many fascinating interview segments of Scott and Seth Avett, their families, their five bandmates and their support crew. Svengali producer Rick Rubin makes Kramer-like cameos, more serious in tone but inevitably drawing giggles when he grooves hippie-style in bare feet and shorts to the brothers laying down tracks in his Malibu studio.

It’s mostly a glowing portrait of the group, though they touch on hard times, too. “When you name a song ‘Divorce Separation Blues,’ there’s really only one thing it can be about,” Seth Avett says in a short section that deals with his split from his wife a few years ago. We see his new partner, actress Jennifer Carpenter, who becomes the mother to his first child by the film’s end; but ultimately there’s not much revealed about the dark side of what happened, despite allusions to it. (This isn’t “Scandal of the Avett Brothers,” it’s a “Portrait.”)

It’s easier to engage absolute sympathies with longtime bassist Bob Crawford’s fight to keep his young daughter Hallie alive after she was diagnosed with a malignant brain tumor in 2011. Seth speaks eloquently about how the experience made everyone in the band stronger: “It pushed all of us to rise to the occasion.” Footage of Hallie in a wheelchair, but smiling brightly when her parents pick her up from a caregiver, underscore both the heartbreak and the beauty of what they’ve been through.

Crawford is like the Avetts’ rock of Gibraltar: As Scott and Seth go through ups and downs perhaps reflected by their seemingly ever-changing physical appearances (long and short hair, beards and clean-shaven), Bob always looks the same, with sparkling eyes and a supportive smile. The rest of the group also gets a chance to tell how they came into the Avett’s gradually expanding orbit, from cellist Joe Kwon and violinist Tania Elizabeth to drummer Mike Marsh to keyboardist Paul Defiglia, first hired as a bassist fill-in for Crawford when Hallie was diagnosed.

Scott’s primary moment comes late in the film, when he gets almost combatively philosophical about a deeply emotional song they’ve just recorded. But maybe the film’s best scene, and one that the Paramount audience clearly appreciated like it was gold, is from a demo session at North Carolina’s Echo Mountain studio where Scott and Seth are tossing around lyrics and ideas for “I Wish I Was,” which eventually surfaced on last year’s “True Sadness” album. We see the song actually coming into being on screen, a fascinating window into the duo’s songwriting process.

During the post-film Q&A, someone who appreciated the band’s inclinations toward gospel music got a surprise information-leak from the band when he asked if they might ever record more hymn-like material. “It comes out in May — it’s done,” Scott said, revealing that they’d be making a formal announcement very soon about a family record they’ve made with their father, Jim Avett.