Suddenly, audiences everywhere want to have fun.

“It’s okay to want to be the best,” keyboardist Andrew Dost says. “It was about trying to be great, and being comfortable with that.”

His line of thinking feels refreshing after years of watching indie-rock bands play down their ambitions. Ruess has a pet theory that indie rock’s half-throttle approach in the 21st century is the direct result of the millennial nu-metal explosion — which is to say, the Limp Bizkits and the Korns of the world made major-label rock-and-roll dreams seem both tacky and absurd. Since nu-metal’s heyday, the trio — Ruess is 30, Dost is 29, Antonoff is 28 — thinks rock-and-roll is too reserved, stylistically and politically.

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“You don’t want to say too much; you don’t want to make any noise,” Antonoff says. “You just want to play your music and quietly hate yourself.”

Everyone laughs, but he’s serious. As the band ascended to stardom, it has been vocal in its support of gay rights and hopes to establish a nonprofit organization in support of same-sex marriage. All three members of fun. are straight but say they don’t want to sit on the sidelines when it comes to what they think is the defining social issue of our time.

“We’re firm believers in the idea that if you’re not talking about it, you’re part of the problem,” Antonoff says. “We realize we have an audience, and it would be a real shame to waste it.”

That activist spirit could have rubbed off on Antonoff as a teenager in New Jersey’s politicized D.I.Y. punk scene, where he worshiped local legends Lifetime. Meanwhile, in Phoenix, Ruess was upgrading from a steady diet of pop-punk to the emotive rock of Arizona heroes Jimmy Eat World. Dost’s teenage years in northern Michigan were quieter — the radio, the Beatles and the high school band.

Ruess formed the group in 2008, having crossed paths with Antonoff and Dost on the road in his previous troupe, the Format. They called themselves fun. to avoid a lawsuit from Fun, a Swedish metal group. The chemistry came quickly, and the band released its debut album, “Aim and Ignite,” in 2009.

“You can say something silly or something stupid,” Dost says of the group’s songwriting process. “You never know what it’ll inspire in somebody else. Maybe it’s a slight twist that takes something from being ridiculous to being really neat, or really big, or really powerful.”

Onstage at Merriweather on Saturday, every song the band plays sounds neat. And big. And powerful. “We don’t get to do these festivals, so I want to try something,” Ruess says, then attempts to coach the audience through a call-and-response routine. The gates have just opened, fans are still shuffling in, and the response is halfhearted. Those Queen DVDs made it look too easy.

But Ruess smiles and keeps belting. He knows this won’t be his last chance.