Justin Bieber slaps Will Ferrell!
Singing sensation Justin Bieber showed everyone he's a prankster by getting his own back on actor Will Ferrell.
They were both out on the red carpet at the American premiere of the new film, Megamind.
Will was talking about his role in the movie, but Justin remembered a prank the Hollywood star had played on him.
Now it was time for Justin to get his revenge, so he gave the actor a slap round the head while he was being interviewed on TV!