''Letters'' is set on Christmas Eve when Kermit and company mistakenly divert three letters en route to Santa Claus. The error triggers a race against time to bring a happy Christmas to the kids whose wishes were lost in the mail.

Thurman will play Joy, the ticket clerk for the North Pole Airlines. Martin will play a postal worker who stars in the opening scene of the special.

Thurman and Martin join previously cast Whoopi Goldberg, Tony Sirico, Steve Schirripa, Richard Griffiths and Madison Pettis, along with Kermit, Fozzie, Gonzo, Miss Piggy and the rest of the Muppets gang.

The Hollywood Reporter