Dan Reynolds of Imagine Dragons headlines a sold-out Friday night show at the Fillmore. (Josh Sisk/For The Washington Post)

Well, at least they made it work in the end.

For much of their sold-out Friday night show at the Fillmore, Imagine Dragons and their audience seemed to exist in separate spheres. Many fans were in a world where Imagine Dragons was a heart-on-its-sleeve, good-time singalong act; the musicians, meanwhile, occupied a place where Imagine Dragons was a Band to Be Taken Very Seriously.

Formed in 2008, the alt-rock ensemble from Las Vegas has a flair for the dramatic in its live show. After two hours (!) of opening music, the headliners began their long-awaited performance with . . . well, with more drawn-out waiting. As a heavy synth hum hung suspensefully in the darkness, lead vocalist Dan Reynolds took the stage and began beating commandingly on a gigantic, leather-headed drum that towered over him; an extensive, gravely tribal drums-only prelude ensued, which (finally) gave way to “Round and Round.”

The pomp and ceremony of it all was a little surprising, considering that Imagine Dragons first made its presence known in mid-2012 with a happy-go-lucky summer jam — the soaring, mandolin-assisted “It’s Time.” The single snagged a No. 15 slot on the Billboard Hot 100, a Darren Criss cover on “Glee,” and plum spots on TV promos for everything from the Wimbledon tennis championships to the Xbox Kinect. Imagine Dragons’ first full-length album, “Night Visions,” largely sticks to that song’s emotive-but-jaunty formula.

Reynolds, an exuberant, skinny-jeans-clad frontman, could devote a whole show to just his hypnotic, hyperbolic arm choreography: The 25-year-old singer has a limited but well-polished repertoire that includes the outstretched Jesus-on-the-cross pose, the finger-point of conviction and the raised fist of solidarity. It’s a sight to behold, but overlong intros, outros, interludes and breakdowns claimed much of Imagine Dragons’ 90-minute set, killing the momentum generated by stirring renditions of songs like “Tiptoe” and “Hear Me.” Four songs in, a crowd-surfer’s tumble managed to distract the audience from the band playing onstage.

“You guys feel like dancing tonight?!” Reynolds shouted soon afterward. Roughly one-third of the audience felt like dancing tonight. The remainder continued to nod along complacently while Reynolds continued arm-waving and chest-slapping onstage. Something wasn’t transmitting.

But when the band began the darkly infectious, apocalyptic “Radioactive” — the newly released third single from their album — finally the worlds collided. A sea of fans mirrored Reynolds’s rhythmic thrashing onstage, his kingly Messiah arms finally channeling in some electric exultation as the fans bellowed the chorus on his behalf. “Radioactive” was precisely where the solemnly theatrical overlapped with the singalongable, and the connection had been made.

By the time Imagine Dragons reached the sugary-sweet, are-you-sure-this-isn’t-from-a-Disney-movie “On Top of the World,” fans were bouncing around joyfully, and Reynolds’s ecstatic delivery of “I’ve been waiting to smile, hey / Been holdin’ it in for a while, hey” sounded true. As one satisfied fan near me sighed happily when the song ended, “That was worth the wait.”

Fetters is a freelance writer.