Armstrong’s bestseller Mischief became the basis for the movie Don't Bother to Knock, which starred a young Marilyn Monroe as a mentally unbalanced baby-sitter. As in Bus Stop, Monroe gives some of her best work, proof that she was more than just a dumb blonde. Mischief would do for babysitters what Psycho would later do for showers. Armstrong’s book started out as a play entitled Little Nell, a wry twist on the idea of the poor young Dickensian heroine. She later changed the story to the now familiar plot of the babysitter who seems reliable, but quickly displays her disturbed nature. The elevator man’s niece is a last minute replacement for the husband’s sister who had a previous engagement. Nell, the babysitter, seems almost mentally challenged at first, but her total lack of morals and concern for the future are slowly revealed to the reader. The book is tightly and expertly written and touches upon the fears of every parent.
Two themes begin to resonate through the works of Armstrong at this point. The first is the idea of the strong woman. Women who might appear slight in appearance, but are made of steel inside have replaced the dependent girls of her first few efforts. In Mischief, Ruth Jones, the child’s mother, is one such heroine. For three quarters of the book, she is merely the companion to her husband who has just wowed an audience with his speech. She is only given a few polite words to murmur in appreciation of her spouse. However, when she feels concern about her daughter being in danger, she immediately rushes to action. She travels through New York City alone and confronts Nell without thought to her own safety.
These women are typically married, happy, and most strong in defending a family member.
The other idea that starts to come through in her work is the idea of a collective unconscious. Armstrong would use the notion in many of her later novels. The subtle idea that a number of characters could share an emotion without discussing it appears in a number of her works. In Mischief, she manages to imbue several of her characters with the realization that they share the guilt for this situation. The child’s brush with danger from the deranged babysitter could have been prevented with intervention from any number of people. All the characters “could” have done something to stop the crescendo of fear and abuse. While none of the characters express this concern verbally, it passes through the thoughts of several of the major characters. The couple staying downstairs might have investigated the crying earlier or the elevator operator might have suggested another babysitter. Thematically tied to the idea of helping each other, Jed, Nell’s date for the evening, even recalls a fight he had with another woman about her desire to give money to the bums on the streets. The notion of contributing to help others runs deep in the novel.