Talk about an odd couple! Reese Witherspoon and Vince Vaughn are co-starring in a new film, Four Christmases, and spies say that the tension between them isn't romantic -- it's pure animosity!

Type A stickler Reese is totally peeved by Vince's more relaxed working style, and vice versa. (We could've seen that coming a mile away.) "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," a source tells the New York Daily News. "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…She's a one-take perfectionist and Vince likes to try it a few different ways. Sometimes Vince will be standing behind her and he has this look on his face that he just wants to kill her!"

Sounds like this movie should be called Four Christmases and a Funeral!