Tucci will play Roderick, the villain of the story and an adviser to the king. The character has plans of taking over Cloister.
Nighy will play Fallon, the leader of the giants. Fallon is two-headed, and will be half-played played by John Kassir.
It has previously been reported that Singer is considering "Kick-Ass" star Aaron Johnson for the title role. That role has not yet been cast.
"Jack the Giant Killer"is a New Line, Legendary and Warner Bros. movie and scheduled to begin filming in March in London.
In the movie, a long-standing peace between humans and giants is threatened when a young farmer tries to rescue a princess who has been kidnapped and taken to the giants' kingdom.
Tucci was nominated for an Academy Award for his portrayal of George Harvey in 2009's "The Lovely Bones."