In the scene on the float platform, Danielle makes a point of how much younger she is than Frances. In fact, Brigitte Auber (b. April 1928) was a year and a half older than Grace Kelly (b. November 1929).
Cary Grant had announced his retirement from acting in February 1953, stating that since the rise of Method actors like Marlon Brando, most people were no longer interested in seeing him. He was also angry at the way Charles Chaplin had been treated by the HUAAC. He was lured out of his retirement to make this film, and thereafter, continued acting for a further 11 years.
On 14 September 1982, Grace Kelly was killed in an automobile accident in Monaco, supposedly on the very same road as her famous chase scene in this film and not far from where she had a picnic scene with Cary Grant. She was 52 years old and lost control of her car after apparently suffering a stroke while at the wheel.
John Robie mentions that as a youth he was in a trapeze group that traveled around Europe. In real life, Cary Grant was in an acrobatic troupe that toured around Europe (and eventually came to America) when he was young.
For the scene between Robie and the insurance agent, when they talk about the cook's sensitive hands, the German version of the movie differs completely from the original. In English, Robie notes she once strangled a German general without a sound, while in German, he says she once caught a lion escaped from a circus with her bare hands.
Ingenuity was required when Hitchcock asked if chase scenes could be filmed from a helicopter as camera mounts did not exist for this purpose at the time. The side door of Sikorsky S-51, UK registered G-AJOV, a former BEA airframe, was removed and the camera jerry-rigged with ropes, wires and cables. These shots were done in July 1954, according to a "Making of" short documentary included in a dvd release.
There is a second reference to Alfred Hitchcock's dislike of eggs. A raw egg is thrown hitting the glass and splattering in the restaurant at the beginning when the kitchen staff believe Cary Grant is responsible for the recent thefts. He is also offered a saucer of milk referring to "cats". Later in the movie, Jessie Royce Landis puts out a cigarette in a sunny-side-up egg.
When everyone is reading the newspaper and Mrs. Stevens says, "Everyone in Philadelphia reads the Bulletin," it's a reference to a long-running ad campaign for that newspaper. The ads were cartoons that showed something extraordinary going on - in a light-hearted vein - but no one sees it because they are all holding the newspaper in front of them. There is always one person who is not reading the Bulletin and trying frantically to get the attention of all the other people. The caption was always the same, "Nearly everyone in Philadelphia reads the Bulletin."
In an early shot, a newspaper article called "Europe's Lighter Side" by Art Buchwald speculates on whether "the Cat" is on the prowl again. Buchwald actually wrote a column by that title for the New York Herald Tribune's European edition early in his career. He left school and moved to Paris in 1948, seven years before this film was released.
The filming of this movie on the French Riviera plays a pivotal role in Wu Ming's novel "54". The action takes place in the springtime of 1954, and nearly all the characters in the novel (including Cary Grant, an Italian American mafioso nicknamed "Steve Cement", and two Parisian gangsters from Rififi (1955)) cross each other's path in Cannes and Nice.
The trivia item below may give away important plot points.
Towards the end, when Francie and her Mother pass a newsstand with a newspaper reporting the burglar's death, Francie's Mother remarks, "in Philadelphia, almost everyone reads the Bulletin." Grace Kelly was a native of Philadelphia.