The mission is called Talk Show Subway Car.
They brought all the chairs, props, people, and equipment onto a subway car and took over, building an authentic late night set.
Evan of course took care of the music as Charlie Todd took charge of the subway riders, quickly turning them into an enthusiastic audience for his fictitious show, Late Night Underground with Pat Cassels.
The show actually looks awesome as Pat asks everyday riders up on stage to feature them on his show. Want more about the crazy prank? Check out the Improv Everywhere article.