The Avett Brothers are enigmatic. Over the band’s 12-year year music career, their sound has refused categorization; Americana, Country, Indie, Folk and Rock don’t justly describe the brothers’ music, but rather elements of their sound. That’s how The Avett Brothers drew their core fan base, which includes directors and producers Judd Apatow (“Girls,” “Knocked Up”) and Michael Bonfiglio, who followed the band through the writing and recording of their 2016 album “True Sadness” and created “May It Last: A Portrait of The Avett Brothers.”

The documentary explores the intersections between music, family, love and heartbreak during a transitional time for the prolific group. The film, the majority of which was filmed in 2014, is clearly a passion project for Apatow, who is well-known for his achievements in comedy as opposed to music documentary. The admiration he has for the subject — and the subject’s love for their music — translates wonderfully on screen and was well received at 2017’s South By Southwest festival.

“True Sadness” itself received critical acclaim, including two Grammy nominations last year. The Avett Brothers have enjoyed a slow and steady ascent to fame since their inception in 2001. Brothers Scott and Seth form the core of the band, writing the songs and performing lead vocals, guitar and piano — Scott also plays banjo. The group’s two other official members are Bob Crawford, the bassist who has been with the brothers since the beginning, and Joe Kwon, a cellist who joined in 2008 during the band’s recording of their fifth album, “Emotionalism.” Mike Marsh (drums) and Tania Elizabeth (violin, backing vocals) also tour with the band, and occasionally, Paul Defiglia, an alumnus of Steinhardt, steps in on keyboards or bass.

“It’s amazing,” said Defiglia of the Avett Brothers. “I don’t know, I’ve never seen two brothers get along that way.”

“Having a band that gets along, that everyone respects each other,” echoed Elizabeth of the group. “It’s not something that I’ve seen, possibly ever.”

Family is at the core of The Avett Brothers in a way that’s not often seen in music. The mutual love and respect between the brothers is constant and is contagious among other band members. Everyone is a part of the family, and not only at work, which is how they humbly view their music careers. When Crawford’s daughter was diagnosed with cancer back in 2011, everyone became deeply involved and closer to one another as they took on the challenge together.

Tragedy, which the band members keenly bare witness to, in The Avett Brothers’ music is a common theme. Songs like “Divorce Separation Blues” and “No Hard Feelings” are brutal to listen to. But not all of it is sad. The music’s deep emotional quality is often the very thing that makes it uplifting, as it acknowledges the difficult feelings that are integral to life. The film pays tribute to everything and everyone that has influences the brothers, from their early grunge days to their country heroes, and captures the difficult-to-define experience of their music. Above all, The Avett Brothers are honest, which is all you can hope for in a musical act.

“May It Last: A Portrait of the Avett Brothers” aired on HBO Monday, Jan. 29 at 8 p.m. and is available for streaming on HBO GO.

Email Natalie Whalen at [email protected].