Johnson, NBC said in a release, left his honeymoon in Kauai early in order to appear on the show, which taped on Tuesday.
“This is real,” Clarkson said in a clip released on Wednesday. “His wife probably hates me.”
He even showed up carrying a flower for Clarkson and received a long, excited welcome from Clarkson’s audience.
Hart was a passenger in a car accident on Sunday in Calabasas, California.
Hart and the driver of the vehicle involved in the single-car wreck sustained “major back injuries,” according to a California Highway Patrol incident report.
Hart’s wife, Eniko Parrish, has said the actor is “going to be just fine.”
Hart and Johnson have appeared in movies like “Central Intelligence” and “Jumanji” together. Off screen, they frequently tease each other on social media.
“The Kelly Clarkson” show debuts Monday, September 9.