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There are always surprises while filming a movie, but this one came as a serious shock to Armie Hammer

In the actor's upcoming film The Man From U.N.C.L.E., Alicia Vikanderslaps her studly co-star and, it turns out, Hammer had no idea it was coming when Vikander pulled the surprise attack during shooting! 

"Well, it wasn't really [a choice], it was the director telling me to do it," the 26-year-old stunner said of the highly anticipated moment in the flick, adding that director Guy Ritchie whispered in her ear and told her to hit her co-star.

In the movie, an adaptation of the 1960s television series, Henry Cavillplays a (smokin' hot) CIA agent who teams up with a KGB spy (Hammer) to bring down an Italian criminal organization. Vikander plays the daughter of a German scientist who is key to infiltrating the bad guys.

VIDEO: Did Henry Cavill get to keep his costumes from the film?

Alicia Vikander, Armie Hammer

David M. Benett/Dave Benett/WireImage

"There were a lot of scenes where Guy just kinda came up and whispered something in your ear," Vikander recalled. "It [the slap] was already in rehearsal when we block out the scenes." 

After the slap was a success in rehearsals, she says that Hammer "had a month to prepare himself for it"; co-star Elizabeth Debicki then chimed in, adding, "A month of therapy." 

"It was still a surprise when it came the first time," Vikander notes, seemingly proud she was able to pull off the stunt. 

Last month, the Swedish actress also opened up about the forthcoming film in an interview with E! News and admitted she apologized to her co-star after the unexpected slap. 

READ: 5 Things to Know About Alicia Vikander

Armie Hammer, Henry Cavill, CinemaCon

Michael Tran/FilmMagic

"[Armie] got a bit surprised the first time I did it," Vikander said. "The first time I did it, I was like, 'I'm sorry.'"

In spite of the slap, Hammer is still a big fan of the director. 

"I think every scene we have in this movie has a certain flair and style to it that really only Guy Ritchie can bring to it," he said at Comic-Con. 

The Man From U.N.C.L.E. hits theaters Aug. 14. 

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