WATCH: How Usher and Edgar Ramirez Got in Fighting Shape to Play Boxers Sugar Ray Leonard and Roberto Duran in 'Hands of Stone'

The two talk paleo diets and swapping punches while paying homage to Raging Bull's Robert De Niro

Photo: Dave Benett/Getty

While some might find getting in shape to play professional boxers daunting, for Usher, it was a “labor of love.”

Hands of Stone stars Usher and Edgar Ramirez sat down with PEOPLE at the Cannes Film Festival to discuss how they physically prepped to play legendary boxers Sugar Ray Leonard and Roberto Duran in the upcoming film.

Usher, 37, said figuring out their diets and workout regimens was one of the first things to bond the two actors.

“The first thing we connected on was ‘How are you losing the weight and how are you holding it?’ ” he explained, adding that Ramirez “made some suggestions.”

“I was doing paleo, which is eating like a caveman pretty much,” said Ramirez, 39.

And while getting into fighting shape was something new and challenging for the pair, it was something their costar Robert De Niro could certainly relate to: The actor famously starred in the landmark 1980 film Raging Bull.

“I certainly understood what they were going through,” he said. “It’s murder when you’re trying to stay in shape and keep your weight down.”

“I thought they were great,” added De Niro.

Hands of Stone hits theaters Aug. 26.