Backstage at Stubb's.

Backstage at Stubb’s.

Sons of an Illustrious Father was playing their electrifying track “Extraordinary Rendition” as a line down the block inched into Maggie Mae’s in Austin. The song set the tone: a thumping bass line played on a keyboard by Josh Aubin (bearded, wearing a speedsuit), alternating with a deliberate hiss on the hi-hat. The back and forth rhythm was intensified by the chords from guitarist Lilah Larson, fierce in black jeans, black T-shirt, Joan Jett cool. Over the top were the plaintive lyrics from the drummer, who wore what I believe was a solid-tone green dashiki but read from the floor like a boxer’s slick robe. The band treated these outfits as uniform, and wore them at every show at SXSW.

The woman behind me leaned over and said, “Isn’t the drummer the kid from The Perks of Being a Wallflower?” He is. You might recognize Ezra Miller from the hilarious cameo (and quirky sex choices) in Amy Schumer’s Trainwreck, or his breathtaking, menacing turn as the malevolent Kevin in We Need to Talk About Kevin, playing opposite the inimitable Tilda Swinton. (The memory of a scene involving fingernails still gives me chills.) If Sons of an Illustrious Father are based “loosely” in Brooklyn, it may have to do with the fact that Ezra is often elsewhere, shooting a movie. He has a cameo as the Flash in “Batman v Superman: Dawn of Justice,” which opened this weekend, and is set to play the Flash in his own movie for 2018.

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No Hollywood pretense was on display at SXSW. The band changed leaders as the set rolled over: Larson did considerable backup howling but also sang lead in a strong alto. Aubin’s keys moved from molar-quaking bass— heavy enough to rattle the wall— to ragtime, to near-honkey tonk. A later song could have been country were it not for the distortion. Instead of introducing themselves, Miller said, “We are a band, here with other bands, in this band place,” which got a laugh. For their finale, before dropping into “Post-Future,” a low-fi anthem sung by Aubin, they sang a capella in three-part harmony from a song by Sweet Honey in the Rock; a lyric written by Dr. Bernice Johnson Reagon as a tribute to the civil rights leader Ella Baker:

We who believe in freedom cannot rest
We who believe in freedom cannot rest until it comes
Until the killing of Black men, Black mothers’ sons
Is as important as the killing of White men, White mothers’ sons

As the band broke down, it was obvious that Sons of an Illustrious Father had just won 20 new fans, which seemed more successful than so many South-by gigs— a room of 50 just looking at their phones. Was that rock? Soulful rock? The drummer— whether he knows it or not— is a soul singer, but the band’s swerves into folk Americana made the genre hard to pin— which is the best kind of music. (The band call it “future folk” or “heavy meadow.”)

Sons of an Illustrious Father just released a new record, Revol, on Big Picnic, which is a stylish example of what they’re capable of in the studio. The credits list all three members playing many instruments, and Larson’s voice stands out as cuttingly addictive. There’s also a comic book by Larson and Audin to go with the single “Conquest.”

Miller and Larson met in middle school, a school Larson said was “a disaster,” and they recognized each other as kindred spirits. They’ve been thick as thieves ever since. They originally booked Aubin six years ago merely as a touring bassist, but after running around with him they told him they were keeping him. Now the three have enough history to feel like their own clique with their own argot (they are fans of the expression “that’s real”) and an easy, humorous manner with each other. You can see it on stage: they’re having a blast.

Sons were also on the bill at Rachael Ray’s Feedback Saturday, opening for Naughty by Nature, George Clinton Parliament & Funkadelic, and—wait for it— “Stephan Jenkins of Third Eye Blind” (that’s how he was billed). They went on at 10:30 am, and if the crowd was riled up and ready for it, there was good reason. This was a free show for Austinites, with free food from Rachael Ray and free beer from Blue Moon, with a line that extended many blocks down the otherwise empty drag of Red River. I learned that the young woman next to me had been in line since 6:30 am—this crowd was happy to be inside, as it was 50 degrees out, and that’s cold in Texas.

The band miraculously got themselves up for the challenge, opening by sitting on the floor and singing to Larson’s guitar accompaniment, building to a crescendo—Miller’s veins standing out as he slapped the stage. Once in position behind the kit, Miller said, “Good morning,” with a wild grin, and Larson, with a sly, knowing one, said, “It’s almost over, guys.”

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On “Extraordinary Rendition,” Miller’s voice was almost cracking, deliberately, and he sounded like no one so much as Joe Cocker, intoning, “You’re beautiful.” On “Because,” light coming through the high window behind the Stubb’s stage made Larson look angelic while sounding almost demonic. Miller: “We’re your fun morning band— bunch of songs about breakup and hell!” The crowd laughed, totally hooked.

“Very Few Dancers” features Aubin with a funky, bouncy organ, and Miller is almost rapping— it is not natural to spit that many words while drumming that well— before slowing into the bridge with a bass boom and Larson’s space-age guitar. And for “Opposite Of Love,” Aubin picked up an electric bass and Miller hunched forward to slap a ride/rim combo, grinning devilishly, as Larson built to an insane howl.

Here, “Post Future” truly landed, as Aubin’s mellow baritone led the refrain, “I’m livin’ in a world that makes too much sense to me.” The crowd went apeshit, and after the band drifted into the bar area it seemed like everyone in the room came up to congratulate them on their set— and to ask, again, the name of the band.

After being led into the VIP backstage area— Airstream trailers, bar, spread of food, guys from Parliament snacking— their eyes popped, and the band admitted they were much more used to punk DIY venues. It was barely 11 am, and they had another show to do at the other end of the night.

That show, at Cheer Up Charlies, didn’t happen until after 1 am. They had told me they were playing a “weird” set, and they opened with “Post Future”; ended with the guitar-backed threefold sing-along, standing in the middle of the audience. And in the middle, they full-on swapped instruments, Larson grabbing the bass, Aubin grabbing the guitar, and Miller taking the keys while Aubin played drum pads. It was late on the last day of South-by, and the crowd was drunk, raucous, and yet wildly appreciative. That the band attempted stunt tricks like a cappella on a crowd too hammered to stand seemed crazy—yet they pulled it off. What next?

And that’s the scary part: if Miller is about to graduate to the Hollywood A-list, what happens to Sons of an Illustrious Father? Will the band become a casualty of Ezra Miller’s successful acting career?

We hope not. We’re looking forward to seeing them come home to play Brooklyn when “The Flash” is in the can. That’s real.

Bradley Spinelli is the author of “Killing Williamsburg” and the writer/director of “#AnnieHall.”