Joaquin Phoenix slapped his co-star Vanessa Kirby on the set of their upcoming movie "Napoleon" after the two made an agreement to "shock each other" during filming.

The 48-year-old Academy Award winner, who plays Napoleon Bonaparte, and the 35-year-old "Mission: Impossible – Dead Reckoning Part One" actress, who portrays his wife Josephine de Beauharnais, opened up to Empire magazine about how they built trust while shooting the Ridley Scott-directed historical drama.

Ahead of filming an intense scene, the two had a discussion in which they agreed to give each other free rein to improvise when it came to physical interactions.

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Joaquin Phoenix slapped his "Napoleon" co-star Vanessa Kirby after the two made an agreement to "shock each other" during filming. (Getty Images)

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"She said, ‘Look, whatever you feel, you can do.’ I said, ‘Same thing with you.’ She said, ‘You can slap me, you can grab me, you can pull me, you can kiss me, whatever it is,’" Phoenix remembered.

He continued, "We had this agreement that we were going to surprise each other and try and create moments that weren’t there, because both of us wanted to avoid the cliché of the period drama.

"And by that I mean moments that are well-orchestrated and designed."

During the scene, the emperor and his wife — who shared a passionate, volatile relationship — are divorcing in a grand ceremony that was historically documented. 

"We were using the real words from their divorce in the church," Kirby explained. "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."

After the duo agreed to ad-lib, they would deviate from the screenplay in certain moments, with one of those times being the unscripted slap.

"We encouraged each other, demanded of each other, to challenge ourselves to shock each other in moments," the "Joker" actor told Empire. "And that’s what came out of that, that moment."

"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,'" Kirby added. 

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Written by David Scarpa, "Napoleon" is "spectacle-filled action epic that details the checkered rise and fall of the iconic French Emperor Napoleon Bonaparte," an official synopsis for the Apple+ film read.

It continued, "Against a stunning backdrop of large-scale filmmaking orchestrated by legendary director Ridley Scott, the film captures Bonaparte's relentless journey to power through the prism of his addictive, volatile relationship with his one true love, Josephine, showcasing his visionary military and political tactics against some of the most dynamic practical battle sequences ever filmed."

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"Napoleon" also marks the first time that Scott and Phoenix have reunited on a project since the Academy Award-winning 2000 historical drama "Gladiator."  

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Joaquin Phoenix and Vanessa Kirby portray Napoleon Bonaparte and his wife Josephine de Beauharnais in Ridley Scott's upcoming historical drama "Napoleon." (Apple)

During the Empire interview, Phoenix admitted that he and Kirby "never got to the bottom" of the complex relationship between Napoleon and Josephine.

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"I don’t know if you can call it love," he noted. "I don’t know what it was."

"Napoleon" is set to premiere in theaters Nov. 22.