Teen Idols -- Then & Now!

Backstreet's back -- with James Corden -- during a "Late Late Show" sketch.

James Corden went from late-night host to full-fledged pop star on Tuesday night, as he joined the Backstreet Boys on stage at their Las Vegas residency.

After he invited them on his show back in 2016, he felt it was right for them to return the favor two years later -- and let him be part of their show for a night. The idea wasn't exactly met with open arms.

"I don't know how they would feel if you were up there," said Kevin Richardson, especially after Corden floated the idea of not just joining them, but replacing one of the guys for the evening. "What is being a Backstreet Boy?" asked the host, rhetorically, "It's wearing a fedora and dancing on tarmac."

After taking a blind vote to decide who would have to sit out the show, Brian Littrell got the boot. FYI: Corden was the only one who actually voted.

From there, he went through BSB bootcamp, learning how to be sexier with his dance moves, before admitting the one part of the show would literally make him "s--t my pants."

It all led up to a performance of "Larger Than Life," the opening number for the night, where Corden held his own against Nick Carter, AJ McLean, Howie Dorough, Richardson and, yes, even Littrell, who was allowed on stage after the other guys made a grander entrance.

But Corden's time on stage was brief, as he could "barely breathe" after just one song. According to the guys, they still had 22 more to go. James' reaction: "Well, then I quit the band."

It's probably for the best.

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