Michael Cera had to lie down after Rihanna slapped him.The 'Stay' singer gave the 25-year-old actor permission to smack her butt during a scene for their new movie, 'This Is the End,' if she could slap him in the face, and according to their director and co-star Seth Rogen, Michael had to lie down in his trailer after she hit him.Seth told Ryan Seacrest on his KIIS-FM radio show on Friday (Jun 13): "We did maybe three or four [takes] and he asked her if he could actually slap her butt and she said he could if she could actually slap him back in the face which she did. And I think one of the times she got him in the ear and it wacked off his equilibrium and he had to go lie down in his trailer for a little bit."The 'Super Bad' star wrote and directed the new apocalyptic comedy in which his friends Michael, James Franco, Jonah Hill, Danny McBride, Jason Segel and Paul Rudd all play ridiculous, exaggerated versions of themselves.He said: "For years, actually, we've been trying to come up with a way to do that ... We've always been fans, I mean 'Seinfeld' did it and 'Larry Saunders Show.' We always loved 'Curb Your Enthusiasm' and we just always thought what would be funny is kind of combining that with a weird supernatural element [which we] made a short film about the world ending, in 2006 and ... we thought maybe we can combine those two ideas and make the strangest movie of all time."View screenshots from the movie's trailer in the gallery below!