Liev Schreiber found working with Naomi Watts on 'Chuck' to be a "treat".
The 49-year-old actor dated Naomi, 48, for 11 years until September 2016 - but their split didn't stop Liev from relishing his time on set with the actress.
He said: "I remember the first day that she came out of the trailer in the leopard print with the fake boobs and the hair.
"She just went for it. I just knew it was going to be fun."
Liev - who has Alexander, nine, and Samuel, eight, with his ex-partner - said 'Chuck' afforded Naomi the opportunity to showcase her comedic talents.
He told People magazine: "When I saw that 'St. Vincent' movie that she did with Bill Murray, Naomi has such incredible untapped comedy potential. She brought some of that to this film and a real compassion."
The movie follows a bartender-turned-boxer called Chuck Wepner who gets the opportunity to fight Muhammad Ali, and the role meant Liev was forced to transform his physical appearance.
He explained: "Fortunately I had been boxing and training a lot for 'Ray Donovan', so it was a kind of easy transition to make.
"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."
Despite his pride at making the movie, Liev admitted he was reluctant to expose his children to such a violent film.
Liev - who has remained close to Naomi since their split last year - reflected: "I'm careful about that because they're still really little. But we have some fights together."