EXCLUSIVE: How 'Late Late Show' Got Gwen Stefani, George Clooney and Julia Roberts for Epic 'Carpool Karaoke'

ET digs into how the most epic Carpool Karaoke came about, and more importantly, how it was kept a secret!

The Late Late Show With James Corden treated fans to the most epic edition of "Carpool Karaoke" yet on Wednesday night when surprise guests George Clooney and Julia Roberts joined Corden and Gwen Stefani for the joyride. Now, ET has details on how the star-studded grouping came about.

Wednesday's segment started out well enough with Stefani as the previously announced guest. The No Doubt frontwoman and the talk show host were having a pleasant drive through Los Angeles, singing some of Stefani's classic tunes, when they hit a snag. Corden feigned frustration at coming up on a section of the "carpool" lane that supposedly required four passengers per vehicle.

WATCH: George Clooney and Julia Roberts Crash Gwen Stefani's 'Carpool Karaoke'

Luckily, Corden was able to call on Clooney, and in turn, Clooney was able to get in touch with Roberts to meet the four-passenger requirement and at the same time take "Carpool Karaoke" to another level.

ET's Nischelle Turner spoke to Late Late Show producers Rob Crabbe and Ben Winston, who explained that booking the two Oscar winners was more an example of "stars aligning" than a plan to get them all on the same schedule.

"I think it was Julia who reached out," Crabbe explained. "She mentioned she'd love to do 'Carpool Karaoke' if we were ever going to do one with someone like Gwen Stefani ... and we happened to be doing one with Gwen Stefani in the next two days."

WATCH: James Corden Reveals His Dream Carpool Karaoke Guests

Winston added that while Roberts and Stefani aren't close friends, Roberts is a big fan of the GRAMMY winner's work.

"She turned up to the carpool wearing a Gwen Stefani shirt!" Winston said. "What was lovely is that they're all huge fans of each other. George was a massive fan of Gwen. Gwen was a massive fan of those two, and obviously all of them, luckily, were big fans of 'Carpool Karaoke.'"

The segment was shot on Wednesday, April 13, which meant they had to keep a lid on the surprise for three weeks. So how did they keep their secret safe?

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In order to shield the stars from paparazzi lenses, Corden and Stefani picked up Clooney and Roberts at the Los Angeles studio where The Late Late Show tapes. Crabbe added that tint on the back windows helped keep the stars out of view of prying eyes.

Check out the video below to see the quartet in action.