The "Stuck in the Middle" star joins Brooklynn Prince and Angelina Jolie, who is also producing the project, according to The Hollywood Reporter.
The film is an adaptation of the Newbery Medal-winning book written by Katherine Applegate and illustrated by Patricia Castelao.
Published in 2011, the book centers on a silverback gorilla named Ivan who lives in a cage in a shopping mall along with an elephant named Stella and a stray dog called Bob.
Greenblatt will play one of the human leads while Jolie will voice Stella.
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