The character of Henry Orient was inspired by real-life concert pianist Oscar Levant. Nora Johnson, who wrote the novel on which the movie was based (and co-wrote the screenplay with her father, Nunnally Johnson), said that she and a friend had a crush on the rather homely Levant when they were schoolgirls.
The phone Henry Orient (Peter Sellers) used in his bedroom is called a Ericofon, made by L.M. Ericsson of Sweden. This is one of the very few foreign phones allowed in the U.S. at the time of filming by the then telephone company, Bell Telephone, which held a monopoly on telephone service and telephone equipment in the US. Bell Telephone felt so threatened by the unique European design (and possible mass intrusion into "their" telephone network) that they designed the "Trimline" phone as a countermeasure.
Included among the American Film Institute's 2000 list of the 500 movies nominated for the Top 100 Funniest American Movies.
Director George Roy Hill handpicked 16-year-old model Walker from hundreds of actresses who auditioned for the role of "Val." The filmmakers were so impressed with her performance that they reshaped the film during editing to focus more on her character, and shot the scene of her walking through a snowy Central Park months after production had wrapped. According to Colapinto, in the 2000s Walker revealed through a series of posts on IMDb that she and Hill fell in love during the filming, and that the relationship lasted throughout most of Walker's senior year in high school, despite the fact that Hill was married with children and, at age 44, was nearly 30 years older than Walker. Walker claimed that the resulting Hollywood gossip made others reluctant to cast her and contributed to her decision to stop acting in the early 1970's.