Michael Cera had to ask permission to slap Rihanna's butt in 'This Is The End'

In the trailer for "This Is The End," actor Michael Cera gets beaten up by Rihanna after slapping her butt at a party.

Cera had to ask for her permission first, he told GQ. Originally, the two were going to fake the slaps. After a few takes, Cera said that it would be better if he could really smack her.

Rihanna agreed, but only if she could really slap him back. Evan Goldberg, fellow "This Is The End" star, told the story.

"Michael said to all of us, 'I’m going to ask her if I can slap her butt for real. I think it will make the joke way funnier.' And we were like, 'Yeah, totally go for it. Go nuts.' And so he asked permission to do the butt slap, and she said, 'You can do it, but I’m coming back way harder.'"

Rihanna hit him back more than six times on set, Seth Rogen said.

Goldberg said he had "never seen a person get slapped that hard in real life."