The trainer, Jo Vaughan, had about 12 weeks to prep the cats. A significant portion was spent familiarizing them with being carried and walking on leashes. “If there’s two things cats don’t like,” Vaughan said, “one is being on a harness and lead, and two is being carried.”

The cats weren’t the only ones who had to adapt to something they didn’t like: When filming began, Nyong’o, who won an Oscar in 2014 for her role in “12 Years a Slave,” was afraid of the creatures. Sarnoski said she spent about an hour before shooting each day doing “cat time” to get to know Schnitzel and Nico.

Still, no matter their training, which also consisted of practicing water scenes in a pool designed for dogs, the cats decided what they were, and were not, willing to do. While shooting a scene set in a flooded subway station in a water tank, Sarnoski said, one of the cats bailed from its makeshift raft. “Joe Quinn, I think, was the one that swam to the cat and was like, ‘We’ll get you out,’” he said. Another challenge was capturing a cat when the camera was in motion. “If you’re on a Steadicam following the cat,” Sarnoski said, “the cat’s always going to be like, ‘What is that thing behind me?’”

Frodo remains catlike throughout the film, occasionally becoming distracted and wandering off. But he comes to represent the human characters’ desire to protect one another. As a service cat, he offers Sam solace — and provides it for the audience as well. It’s easy to see why Nyong’o ultimately became a cat person, cuddling her furry co-stars.

Once filming wrapped, and Nyong’o had to part with Schnitzel and Nico, she adopted a cat, who is orange and named Yoyo. “Everyone should have a cat,” Sarnoski said. He ended up getting one, too.