Not everyone knew what a hit James Corden's Carpool Karaoke would become.
The Late Late Show host recalls the initial process of booking the bit, when he couldn't even get "fifth rate" artists to sign on for an appearance.
"If you imagine an artist, just think of any artist in the world, they said no to it. I mean, everyone," Corden said in an interview on Howard Stern's SiriusXM radio show. "I'm talking, like, who's the dudes who did 'Cotton Eye Joe'? We were in those sorts of territories. ... And they're like, 'I'm not doing that.'"
WATCH: James Corden and Jennifer Lopez Text Leonardo DiCaprio in New Primetime 'Carpool Karaoke'
Then came Mariah Carey, who ramped up the diva factor for the very first version of the segment. Corden explains that he first won over a woman from Carey's record label, who in-turn pitched the idea to Carey. The singer agreed to participate as promo for her Las Vegas residency.
"She came out and said, 'Oh, I'm not singing today,' and she went, 'We'll just drive around and have a chat,'" Corden recalled with a laugh. "I went, 'Oh, right. It's just, we call it Carpool Karaoke, so without that...' And then I thought, I've just gotta go. I've just gotta jump into it. Even if this bit is just me singing to Mariah Carey, it's still on the show. It may not be the recurring bit that we hope it is, but it's still probably on the show."
Indeed, it only took one minute for Carey to declare "I'm not singing today, I was up all night!" in the edited package. Watch below.
WATCH: James Corden Reveals His Dream 'Carpool Karaoke' Guests
The 46-year-old superstar eventually gave in to Corden's charm, gamely singing along to "Always Be My Baby," "Vision of Love," "Thirsty," and Ol' Dirty Bastard's "Fantasy," even showing off her British accent between tracks.
"To her credit, she got involved and it was wonderful," Corden said.
Only a few short months after its debut, Carpool Karaoke has already become a celeb favorite, booking superstars like Adele, Justin Bieber, Jennifer Lopez, Elton John, Sia and more. Even Julia Roberts and George Clooney took a spin in Corden's car recently, crashing Gwen Stefani's session. See how the epic shoot came together in the player below.
WATCH: How 'Late Late Show' Got Gwen Stefani, George Clooney and Julia Roberts Together for 'Carpool Karaoke'