Reese Witherspoon & Vince Vaughn – Four Christmases (2008)

These two made the perfect couple in this hilarious holiday hit about, well, relationships. Yet, it turns out that Reese and Vaughn were anything but close when the cameras stopped rolling. Sources say the two were constantly clashing over style. Vaughn preferred his relaxed and impromptu method, while Reese always insisted on being more prepared and rehearsed for each scene. At least for the moments of tension in the plot, the two must have filmed those scenes easily!