According to reports, Reese Witherspoon and Vince Vaughn hated each other during filming, though nobody knows why. During  production, a number of stories leaked to the press about the tension on  set. The Daily News quoted "spies" who gossiped about how the stars'  personalities clashed: "Vince rolls onto set in the morning looking like  he just came in from a night out, while Reese will arrive early looking  camera-ready," said their San Francisco source. "Then Reese tries to force Vince into blocking out each scene and running through their lines  as Vince tries to convince her that he's an ad-libber and wants to play  around and see where the scene goes." It got so bad that Witherspoon refused to film a steamy sex scene with Vaughn