Rihanna gave Michael Cera permission to slap her in "This is the End" as long as she was allowed to hit back. YouTube screencap If you've watched the trailer for upcoming apocalypse comedy "This Is The End," you've most likely seen Michael Cera get smacked by Rihanna after slapping her behind at a party.

The hits taken by Cera aren't fake.

She really did slap him, and quite a few times.

In an interview with GQ, James Franco and the cast of the picture revealed how the scene with Rihanna came about.

Cera asked permission of course.

Costar Evan Goldberg gave a blow by blow of how it went down:

"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.'"

According to star Seth Rogen, Rihanna hit the actor at least six times. One of the slaps was so hard that it actually temporarily impaired his hearing.

"It was like a flash bomb went off," said Cera. "There was a high-pitched tone ringing in my ear, and I didn't know where I was."

Goldberg added that Cera needed to take a break.

"He was like, 'No, will you help me sit down? My balance is kind of gone,'" said Goldberg. "I've genuinely never seen a person get slapped that hard in real life."

The movie looks like the funniest of the summer — and maybe the year — and is jam-packed with stars playing versions of themselves.

"This is the End" hits theaters June 12.