Peter Jackson, director of "The Lord of the Rings" trilogy, will also direct the two-part prequel that features Tony Award winner Sir Ian McKellen (Amadeus) and Lee Pace (The Normal Heart) among the cast.
Luke Evans and Evangeline Lilly have also recently joined "The Hobbit" as Bard and a woodland elf named Tauriel, respectively. Humphries will play the Goblin King, and Cumberbatch will voice the dragon Smaug.
Humphries has been seen on Broadway in Oliver!, Dame Edna: The Royal Tour, Dame Edna: Back with a Vengeance and All About Me. Cumberbatch, who recently alternated the roles of Victor Frankenstein and the Creature in Mary Shelly's Frankenstein at National's Olivier Theatre, has also been seen in the West End in After the Dance and Hedda Gabler.