Water for Elephants
In Water for Elephants, Reese Witherspoon actually declined to kiss Robert Pattinson. It wasn't because she didn't like him, but more so because he was sick with a nasty cold. For some reason, they still made them go through with the scene, snot and all!
Martin
Martin Lawrence and Tisha Campbell played a loving couple in the 90s tv show Martin. The two had amazing on-screen chemistry, but something went wrong along the way as the seasons went on. Campbell began to refuse to kiss Lawrence anymore and even filed a harassment lawsuit against him. Eventually, she refused to film with him altogether.