It’s an extraordinary experience to observe Andy Serkis and his fellow motion-capture castmates-as-primates perform on the set of War for the Planet of the Apes. At first there’s the pure visual oddity of watching loose-limbed actors decked out in bodysuits, helmets, and dotted-up faces communicate with varying command of the English language.

In this case, the ensemble is on a soundstage where an abandoned ski lodge has been meticulously constructed. Serkis (who returns as ape leader Caesar) is flanked by fellow series mainstays Karin Konoval (Maurice) and Terry Notary (Rocket) as they confront a new face: Steve Zahn (Bad Ape). “Human get sick, ape get smart,” Zahn tells them just above a whisper with deliberately fractured delivery. “But not me. I run!”

Eventually, something transformative happens. We forget we’re watching human beings. The CGI is still months away, but it already feels like we’re in the presence of apes. “That’s interesting, and very much the point,” Serkis laughed when we relayed our revelation to him in his trailer later that afternoon. But it’s no day at the zoo for the actor. “It’s very painful,” Serkis said of the physicality involved.

It’s Day 18 of the threequel’s 95-day shoot at Mammoth Studios in New West Minister, B.C., about a 30-minute drive from downtown Vancouver. And in a nearby trailer, Zahn, an actor typically known for bringing comic relief and who’s just recently begun his motion-capture debut, is visibly exhausted. “It’s extremely demanding,” he says. “It’s daunting to me personally because I’m coming into something that’s already been rolling for a long time, and these guys are incredible performers. So it’s scary. I was extremely nervous coming into this.”

View photos Karin Konoval, Andy Serkis, Terry Notary, and Michael Adamthwaite filming War for the Planet of the Apes. (Fox) More

Directed by Matt Reeves from a script he co-wrote with Mark Bomback, War for the Planet of the Apes marks “the final chapter” in a trilogy following box-office hits and surprise critical darlings Rise of the Planet of the Apes (2011) and Dawn of the Planet of the Apes (2014). After a drug made the apes increasingly intelligent in Rise and humankind became endangered because of a resulting virus in Dawn, War — as its title implies — will see the two fighting for the survival of their respective species. “There are no winners. It’s just a brutal situation,” Serkis explained.

The “Caesar-centric” story opens with the apes, having lost their enclave in Dawn, taking residence in a hidden fortress behind a waterfall. Humans attack, and something cataclysmic unfolds. This sends Caesar, typically a peace broker between man- and ape-kind, on a path for revenge, with his sights set on the brash military leader, Colonel (Woody Harrelson).

During their subsequent journey, Caesar, Maurice, and Rocket find Zahn’s Bad Ape at the ski lodge. He’s a smaller, slightly unhinged zoo escapee whose existence leads Caesar to believe there could be more apes out there like him.

“It’s like doing theater again, and being in that absurd world,” said Zahn, who compared the motion-capture work to the two years he spent doing experimental productions at the American Repertory Theater in Cambridge, Mass. It was daunting for the actor, but he had some valuable allies at his disposal. Most notably, Serkis, whose work in films like the Lord of the Rings trilogy, King Kong, Star Wars: The Force Awakens, and the Apes films has made him the Brando of Motion Capture (“He’s like royalty,” Zahn said); and Notary, a Cirque du Soleil alum who serves as a mo-cap player-coach, hosting an “Ape Camp” in the lead-up to each installment. (“Terry Notary is so central and so much the heart of this whole thing,” Serkis said. “Not only does he play one of the roles, Rocket, but his responsibility is enormous because he’s watching what everybody else is doing.”)

View photos War for the Planet of the Apes (Fox) More