Joaquin Phoenix, Vanessa Kirby kept it spicy on ‘Napoleon’

Vanessa Kirby and Joaquin Phoenix are acting out on the set of “Napoleon.”

Ridley Scott’s epic biopic, set to be released this November, follows Phoenix as the infamous military general charting a swift and brutal rise to power.

The narrative focuses on Napoleon Bonaparte’s hot-and-cold relationship with his first wife, Joséphine de Beauharnais (Kirby), and the fallout from the couple’s tortuous divorce.

In one particularly heated scene, Phoenix even delivers Kirby an unscripted slap.

Speaking to Empire magazine, the stars claimed that the ability to shock one another was the secret to their success.

“We were using the real words from their divorce in the church,” Kirby, who was nominated for an Oscar for her role in “Pieces of a Woman,” said. “When that happens, you can faithfully go through an archival re-enactment of it and read out the lines and then go home. But we always wanted to surprise each other.”

It isn’t hard to imagine that a dense period drama like “Napoleon” might collapse under its own weight — or collapse under the weight of Bonaparte’s hat — so the actors sought ways to inject some life into their scenes. That meant trusting each other to make creative decisions that might verge on violent.

Kirby explained, “It’s the greatest thing when you have a creative partner and you say, ‘Right, everything’s safe. I’m with you. And we’re gonna go to the dark places together.”

Four-time Oscar nominee and one-time winner Phoenix, meanwhile, gave props to his on-screen paramour for her gumption: “She said, ‘Look, whatever you feel, you can do.’ I said, ‘Same with you.’”

Phoenix elaborated, “She said, ‘You can slap me, you can grab me, you can pull me, you can kiss me, whatever it is.’ So we had this agreement that we were going to surprise each other.”

Director Scott has previously spoken of the unwieldiness of the work he’s attempting to pull together, telling Empire of Bonaparte, “I compare him with Alexander the Great, Adolf Hitler, [and] Stalin.”

“Listen, he’s got a lot of s–t under his belt,” Scott said.

“At the same time,” he continued,”he was remarkable with his courage, and in his can-do and in his dominance. He was extraordinary.”