Sir Paul McCartney performs onstage during Desert Trip at the Empire Polo Field on October 8, 2016 in Indio, California. How?s this for a holiday treat? Jimmy Fallon and his ?Tonight? show house band, the Roots, enlisted members of the cast of ?Sing? -- Scarlett Johannson, Reese Witherspoon, Matthew McConaughey, Seth MacFarlane and Tori Kelly -- for a festive, a cappella take on Paul McCartney?s ?Wonderful Christmastime.? As a bonus, the Beatle himself showed up to help out with the yuletide singalong of the 1986 holiday hit.