Singer/guitarist Robin Pecknold clearly wasn’t feeling well during the Fleet Foxes' Austin City Limits taping on Friday night. The poor guy kept sipping from mugs of hot tea between songs, and wiping the sweat from his face in the chilly room, but that didn’t stop his band from delivering a heavyweight set.

There’s a lot made of the Foxes’ “lush, orchestral” sound (fun game: Google “Fleet Foxes lush” and take a drink for every writer who’s described the band with that word), but live, especially in a room like the Moody Theater at ACL Live, there’s not much mellow about the band. In fact, it’s almost the opposite. While the band’s reputation is as a group of laid-back dudes from the Pacific Northwest who are obsessed with Crosby, Stills, and Nash, their live set is big-time rock and roll.

That’s never been more clear than at the Moody Theater, and especially during the taping of Austin City Limits, when there upper half of the venue is closed off to create an even more intimate space for the performance. It made the material—largely culled from this year’s Helplessness Blues—seem downright rocking. Yeah, the harmonies are still fully intact and, dare we say it, “lush,” but sometimes six people with instruments on one stage just make a hell of a lot of noise. That was clear from about halfway through the set’s opener, “The Plains / Bitter Dancer,” when the song exploded into its big rock stomp. Fleet Foxes’ greatness as a rock band is easy to overlook when you get distracted by the flutes and the harmonies, but the group’s loud three-guitar attack is what makes the violins and a capellas sound so dynamic—it’s all a reminder of just how multi-faceted the band can be.

Pecknold rarely addressed the audience, and when he did, it was mostly about how he was “singing on borrowed time”—a reference to whatever illness he was struggling with—but the story of the set was the dynamism that the band displays. On record, Fleet Foxes sound so well-produced that it’s fair to wonder how that translates to a theater gig. The key is the multi-intrumentalism of most of the band—during the eleven songs they played for Austin City Limits, there were guitars electric and acoustic, drums, an organ, a mandolin, an upright bass, a flute, a saxophone, a slide guitar, a 12-string guitar, tambourines and a violin used by different band members (usually the group’s MVP Morgan Henderson). The sound didn’t really need to translate to a small theater stage; with that many instruments, it could authentically be re-created.

The crowd mostly seemed to get it, too. For Austin City Limits tapings, there’s usually a pretty mixed audience. Some people are there because they adore the band, and others are there because they won a drawing for tickets. As a band with so many breaks and movements in its songs, there were more than a few moments when the crowd would break into applause and cheers during the bridge without realizing that the song wasn’t finished. But that can almost be expected with a band whose songs are as complex as those that Fleet Foxes write. There was also a lot of stomping and clapping along with the song, a nice complement to the driving rhythms that drummer Josh Tillman favors. The powerhouse aspects of the set—that rock and roll energy that may have come as a surprise to those expecting the band to keep it mellow—dominated the room even for people who were obviously unfamiliar with the music.

That’s what makes Austin City Limits work no matter who’s on stage, in the end. The theater’s intimacy, preserved nicely even in its new, multi-million dollar digs, allows a performance to feel like a dialogue, and the Fleet Foxes have a lot to say. The small stage at the Moody Theater gave them a showcase to display everything that they’re good at, which is just about everything—lovely harmonies, gorgeous melodies, big explosions of rock and roll noise and a dizzying array of instruments to show off the complexity of the band’s songs. For all of the talk about the lushness of the group’s sound, there’s an anarchic spirit that comes through in a small room, too, and it makes Fleet Foxes an increasingly interesting band.