The 16-year-old boy tumbled about 25 feet into the pit, where a net broke his fall, and his injuries weren't considered life-threatening, fire department spokesman Steve MacDonald said.
The initial emergency call to firefighters was about a person who had fallen into a zoo's lion pit, MacDonald said.
"You know, you start having these images of animals outside, but once the officials assured us that there were no animals at all, it pretty much became a standard extrication from a hole," he said.
When firefighters showed up, there was a movie set medic with the boy, who was conscious and talking as emergency workers put him on a stretcher before taking him to a hospital.
The boy, MacDonald said, was in an area where he shouldn't have been and wasn't working for the movie, which features James, the former star of the CBS sitcom "The King of Queens," as a zookeeper unlucky at love.
Zoo officials were investigating the afternoon incident, which occurred in an old gorilla exhibit known as the Lion's Den for the movie.
In "The Zookeeper," James' lovelorn character has animal friends that break their code of silence and reveal they can talk in a desperate attempt to help him win the girl of his dreams and prevent him from quitting his job. The movie also features Cher and Sylvester Stallone.
The Franklin Park Zoo, which has hundreds of exotic animal species from around the world in exhibits including a tropical rain forest, the Australian outback and the African savannah, made headlines in July after its operator suggested some animals might have to be destroyed because authorities cut $4 million in state funding.
Gov. Deval Patrick called the claim a scare tactic. He said no animal would be killed or was ever truly threatened with euthanasia. He said the zoo had responded to the budget cut "by spreading inaccurate and incendiary information."