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.
As part of his research for this movie, William Wyler met with the real Maria von Trapp and the Mayor of Salzburg. Wyler was concerned that the local residents would be alarmed at seeing their buildings draped with Nazi flags and seeing stormtroopers in the streets only twenty-five years after the real thing had taken place. The Mayor assured him that the residents had managed to live through the Anschluss the first time and would survive it again. Other city officials were much more resistant to the idea of decorating Salzburg with Nazi colors. They soon changed their minds when the filmmakers said they would use newsreel footage instead. This footage was actually highly incriminating as it showed the Salzburgers openly welcoming the Nazis, something that the proposed scenes for this movie would not do.
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."