Christopher Plummer accidentally said the word "Captain" to Julie Andrews during the argument scene. Despite the error, producer and director Robert Wise thought it was that amusing, and liked it so much, he kept it in the movie.
Christopher Plummer intensely disliked working on this movie. He was known to refer to it as "The Sound of Mucus" or "S&M" and likened working with Dame Julie Andrews to "being hit over the head with a big Valentine's Day card, every day." Nonetheless, he and Andrews remained close friends until his death. Andrews claimed that Plummer's cynicism probably helped his performance and this movie, keeping it from being too sentimental.
Julie Andrews sang "Supercalifragilisticexpialidocious" to the children in the cast to entertain them between shooting. Since Mary Poppins (1964) hadn't yet been released, they just thought she'd made up the song for them.
When Maria is running through the courtyard to the Von Trapp house in "I Have Confidence", she trips. This was an accident. However, producer and director Robert Wise liked this so much that he kept it in the movie. He felt it added to the nervousness of the song and of the character.
Charmian Carr, who played Liesl, and was twenty-two at the time, wrote in her autobiography that she was attracted to the 35-year-old Christopher Plummer, who played her father. Plummer admitted that the feeling was mutual, but insists that it didn't get beyond mere flirtation.
Maria von Trapp: Portraying the elder of the two women in Austrian peasant garb who are in the background as Maria (Dame Julie Andrews) walks through a brick archway during "I Have Confidence."