In the scene below, sisters Betty (Clooney) and Judy (Vera-Ellen) are performing a show when their former landlord shows up with the local sheriff, claiming that they burned a hole in their hotel room rug and owe him money. The girls claim that this is a scam, and they don't have the money to pay in any case. Bob (Crosby) and Phil (Kaye) decide to help the girls: to buy time while Betty and Judy escape out the window in their dressing room, Bob and Phil borrow a few items from the sisters' wardrobe and "perform" one of their songs, lip-syncing to a recording of the girls singing.
According to Rosemary Clooney, this scene wasn't originally in the script. Crosby and Kaye were fooling around one day and Michael Curtiz, the director, thought their clowning was so funny that he wrote it into the movie. They filmed the scene several times, but Danny Kaye's antics kept cracking Bing Crosby up. They eventually managed to get a take without him laughing, but when they compared the film footage later, everyone liked the one in which Crosby is losing it at the end best, so that's what they used in White Christmas.