On Saturday night, [artist id="440"]Weezer[/artist] celebrated Halloween with a concert at New York's Hammerstein Ballroom. It was your typical Weezer show, full of big hooks and lung-busting sing-alongs, but when it came time for the group to play new single [article id="1618774"]"(If You're Wondering If I Want You To) I Want You To,"[/article] they welcomed a special guest onstage: "Gossip Girl" star and "Good Girls Go Bad" singer [artist id="3208054"]Leighton Meester[/artist].

She stormed the stage and duetted with Cuomo on the song, which sounded extremely polished but was mostly off-the-cuff.

"We invited her down, and she was in rehearsal -- wait, did she even come to rehearsal? No, she didn't!" Cuomo told MTV News, trying to recollect how the events shook out. "We just worked on it in the dressing room right before the show. 'You sing this line, now I'll sing that line,' just like that. It's best when there's a little rehearsal, so that it can still feel spontaneous."

For most of their 15-year existence, Weezer have pretty much kept to themselves, rarely going outside of their immediate universe. But that has all changed with [article id="1618936"]Raditude,[/article] the band's sixth album, which drops Tuesday. The spirit of collaboration has led to a guest spot from [article id="1622477"]Lil Wayne on the song "Can't Stop Partying,"[/article] and Cuomo has recently spent time with Adam Lambert and [article id="1623880"]Katy Perry[/article] on music. He credits a tiny tour that brought them closer to the fans.

"We did a tiny tour called the [article id="1588728"]Hootenanny Tour[/article]," he explained. "We invited fans to show up with instruments, and we had to figure out how to jam with these people we've never met with instruments I had never seen before. It was really challenging and exciting."

So exciting, in fact, that Weezer repeated the feat on Monday's (November 2) episode of "It's On With Alexa Chung," which featured yet another duet between Cuomo and Meester on "I Want You To."