Sarah Shatz
May 03, 2017 04:19 PM

Liev Schreiber and ex Naomi Watts have remained friendly and hugely supportive of each other despite their split, and the actor says that working with her on their new movie Chuck was “a treat.”

Of course, her character’s outfits in the film might have had something to do with that: “I remember the first day that she came out of the trailer in the leopard print with the fake boobs and the hair,” he tells PEOPLE in this week’s issue, adding, “She just went for it. I just knew it was going to be fun.”

The former couple and parents of two — Alexander (Sasha) Pete Schreiber, 9, and Samuel Kai Schreiber, 8 — announced their split in September after 11 years together. They’ve remained close since, wishing each other well on social media and even sharing a friendly kiss at the film’s Tribeca Film Festival premiere in New York City on Friday.

The duo play husband and wife in the biopic, which tells the story of heavyweight boxer Chuck Wepner. Chuck follows the rise of the bartender-turned-boxer as he gets the chance to fight Muhammad Ali, along with his hard-partying lifestyle that landed the pugilist in jail for cocaine possession. Schreiber takes the lead as Wepner while Watts plays his wife, Linda.

RELATED: Naomi Watts Breaks Silence on Liev Schreiber Split

“We had gone out to see a fight with Chuck and Linda,” Schreiber says of himself and Watts, who picked up quickly on Linda’s fashion and personality.

“When I saw that St. Vincent movie that she did with Bill Murray, Naomi has such incredible untapped comedy potential,” he added of his ex. “She brought some of that to this film and a real compassion.”

Watts wasn’t alone in changing her appearance for the film. As Schreiber explains, he had some bulking up to do before playing the 6’5″, 222-lb. boxing legend. “Fortunately I had been boxing and training a lot for Ray Donovan, so it was a kind of easy transition to make,” he says.

“If anything, I needed to get kind of bigger and fatter because fighters in the ’70s, they had very different physiques than they do today, which I was very happy to oblige them with,” he jokes.

As for whether the former couple will be able to share their joint project with the kids, Schreiber says he’s thinking twice before exposing them to the violence. “I’m careful about that because they’re still really little,” he explained, adding, “But we have some fights together.”

In honor of their parents’ film, Schreiber says, “They made punching-bag balloons and they drew Muhammad Ali and Chuck Wepner’s faces on them and they staged their own boxing match, which I thought was very funny.”

Chuck hits theaters May 6.

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