What began with Daniel Craig in Star Wars: The Force Awakens has now expanded into The Jungle Book, director Jon Favreau tells PEOPLE.
At the Disney and Cinema Society New York screening of The Jungle Book on Thursday, Favreau confirms a few stars did furtively participate in the film – but he's not naming names.
"No," he answers. "Because they didn't want to be credited. But if you're smart, and you listen carefully, you might be able to guess."
The all-star cast officially includes Bill Murray, Ben Kingsley, Idris Elba, Lupita Nyong'o, Scarlett Johansson, Giancarlo Esposito, Christopher Walken, and the late Garry Shandling, but they aren't the only ones who contributed.
"All my kids do little voices,” Favreau tells PEOPLE. "And other people's kids, too. It becomes like a home movie. There are lots of secret people, voices. You can look at the list of names and you'll see."
In the list of cast members, PEOPLE immediately spotted Max Favreau as Young Wolf #2, Madeleine Favreau as Raquel the Rhino (voice) and Brighton Rose [Favreau] as Gray (voice). Naomi Watts and Liev Schreiber's boys also got in on the action, with Alexander "Sasha" Schreiber as Young Wolf #5 and Samuel Kai Schreiber as Young Wolf #7.
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How did his own children do?
"They're great," says Favreau. "A lot of the recording was done when Daddy was at work and they'd come by, and I'd get them to do a line or two. And so it was a fun thing to get them to be wolf voices or puppy voices. We get a kick out of it."
The Jungle Book roars into theaters April 15.