NOT HIS FIRST ANIMAL ROLE

Mr. Zahn lends his unmistakable voice, expressive features and gymnastic physicality to a key character. As Bad Ape, the nervous sidekick to the series’ heroic protagonist Caesar, he gives the film crucial doses of comic relief.

Although Mr. Zahn studied at the American Repertory Theater’s school for acting at Harvard and has spoken in movies as a pig, cat, hawk, shark, bear, chicken and dinosaur, nothing on his resume prepared him for this role, he said.

He had to adjust to the fact that none of the actors looked anything like what they were playing. “I think if we had been doing it with people in ape suits, it would have been easier” for the performers to get into character. “You’d look at them and think: You’re an ape,” he said. “But you were doing it beside people wearing a helmet with a camera, and 51 dots on their face, and a gray unitard. The pressure to become an ape was much greater.”

In free moments between shots, he would go off with stuntmen who had worked on earlier films in the franchise “and pretend to be chimps.”

But while there is well-documented research on how apes look and move, there are no precedents for how they speak. Bad Ape’s sound and cadence, which echo the anxious tenor of the legendary Barney Fife, the bumbling, lovable, deputy sheriff on the 1960s TV comedy “The Andy Griffith Show,”was entirely Mr. Zahn’s creation.

Director Matt Reeves “was very concerned about how I’d talk in relation to the other apes.” His character, a zoo escapee, “lived alone, so you just play it in the moment. If you think about ‘How am I sounding,’ you’re not acting.

“The beauty of acting is you just get lost in it, you play the character, and you hope that it works. You just interact with each other like on any other movie. Except on this one you’re squatting as low as I could do to the ground, and my legs were shaking so hard that I wanted to stop, and Matt was shooting on digital, where a take can last 30 minutes. I was terrified my back was going to go out. It’s crazy.”