The first question Corden asked was, "Never have I ever lied about seeing an X-Men Movie?" The entire cast took a sip.
Jessica Chastain declared: "Never have I ever farted and blamed it on someone else in this group" — and without missing a beat McAvoy responded, "I did it today!"
The whole adventure started when Corden admitted he had spent the entire show's budget on a double decker bus, to give celebrities the "James Corden's Star Star Tour: The Tour that Takes the Stars on Tour."
Among the historic landmarks pointed out by tour guide Corden was the Tesco Express store where "Sean Penn lost his virginity".
The gang enjoyed the ride, singing the politically correct "X-People" song, snapping pictures and laughing ... until they demanded ice cream.
"There's no time for ice cream," Corden told them, which prompted the entire cast to react like unruly four-year-olds until they finally got their way. Chastain even complained that she didn't eat dairy, and said her replacement lollipop was too phallic.
After the ice cream truck pit stop, each of them displayed their "Hidden Talents," which included catching marshmallows, Chewbacca noises, riding a unicycle on a bus, a robin red breast birdsong rendition and a rather tame "Bewitched" spell.
Alexandra Shipp, meanwhile, almost made Chastain vomit by belching the alphabet.
Watch the whole video above. "X-Men: Dark Phoenix" is in theaters now.