In recent years, the Foellinger has grown more ambitious in its summer programming: booking ever larger rock acts of the sort that never would have played the outdoor venue 15 years ago.

But when the Foellinger booked Modest Mouse, it took a leap into the unknown. The band performs Sept. 21 at the amphitheater.

Modest Mouse is one of the most beloved “indie rock” acts (and one of the most stubbornly independent bands of renown) performing today.

More than 20 years after Modest Mouse first released its breakthrough album, The Lonesome Crowded West, the band remains difficult to pin down. It’s a quality that is much loved by Modest Mouse’s most ardent devotees.

The band’s concerts and albums are often described by fans and detractors alike as sloppy and unfocused. The source of much, if not all, of that mutability is Modest Mouse’s lead singer and creative heart, Isaac Brock.

“Mercurial” is an adjective that tends to follow Brock wherever he mercurially goes. Brock rarely grants interviews and when he does acquiesce to them, he tends to be less than forthcoming.

The band has seen a lot of turnover (its original bassist, Eric Judy, left in 2012). Brock has seemed a little defensive at times about all those arrivals and departures, but a reporter with the Manchester Evening News once asked him the secret to keeping a band together and Brock earnestly answered, “The secret is not to fake conversations.”

“When you’re on tour for months,” he said, “don’t try to force conversation for the sake of it.

“Sometimes I can go for days without speaking to someone in the band,” Brock said. “It doesn’t mean anything is wrong, ultimately, it’s good for harmony.”

The departure of Judy was especially jarring. Many fans assumed Judy was in it for the duration.

Brock addressed Judy’s leavetaking on CBS This Morning: “There wasn’t any falling out or anything like that,” he said. “It didn’t go dark. I think just maybe 20-some years was enough time for him.”

Modest Mouse scored a couple of surprise hits in 2004 with “Float On” and “Ocean Breathes Salty.” But Brock is definitely not a guy who analyzes a surprise hit for clues on how to score future hits, a fact he reiterated to The Irish Times.

“Nah, nah, nah,” he said. “I just can’t (expletive) be bothered to worry about that sort of stuff. I worry about things like, ‘Am I being too clear? Am I being too literal in the song?’ I like sharing songs and, in doing so, you have to think about, ‘What’s the best way I can share this experience with people?’, not ‘Will this song sell more albums?’”

Brock told the McClatchy-Tribune News Service that the band doesn’t just release random collections of songs. There is always some overarching idea, even if it remains just out of reach for some listeners.

He lamented the short attention spans of today’s young music aficionados.

“It’s sort of like it was in the ’50s, when it was just singles,” he said. “You’d put out a 7-inch record that goes to radio, and the album wasn’t a big deal. With the way people are picking and choosing songs instead of spending time with a record, are they really enjoying it? … They’re always going with the new taste.”

Asked by the aforementioned Irish daily how he had changed as a person and a musician over the years, Brock responded, “Sadly, I’m less optimistic, definitely (less so than in) the early days of the band. I’m currently more resigned to fatality, but that could just be this week.

“It’s like asking someone who doesn’t have any (expletive) mirror what they look like,” he said. “I’m not allowed the privilege of perspective on this. I’m not really sure how it looks from the outside.

“From the inside, it’s like someone gave me a giant bin of puzzle pieces (in 1996),” Brock said. “It’s the same bin, but occasionally some (expletive) comes by and just dumps some more pieces in there.

“I’m still just trying to sort it out and figure out which puzzle pieces go in the right (expletive) place.”

Not surprisingly, Brock doesn’t give much thought to his own mortality or the legacy he will leave behind.

“I dunno,” he said. “I’ve only got one ticket so I’ve been on the same (expletive) ride this whole time. I’m not sure if it’s a long ride or a short ride. I can’t really say that I’m getting real perspective on anything.

“I’m very aware of the world’s mortality,” Brock said. “I remember being worried about environmental issues as a kid, before it was even a (expletive) topic. Me? I’m just a skin cell on the world, so my mortality doesn’t really mean (expletive) all.”