Due to the suspense, the stars, and the frequent plot twists, many people believe that this is an Alfred Hitchcock film. He was not involved in the making of the film at all. This confusion has prompted fans of the film to call it "the best Hitchcock film that Hitchcock never made".
Cary Grant initially turned down the movie because he felt he would be too much of a predator pursuing the much-younger Audrey Hepburn. In a last-ditch attempt to sign Grant, Peter Stone worked all night on the script and presented it to Grant to look over just once more. Grant gleefully accepted the role, prompting producers to demand to know what Stone had done. Stone had simply moved all of the romantically aggressive lines from Grant's character to Audrey Hepburn's, making her the predator.
After finishing this film, Cary Grant was quoted as saying, "All I want for Christmas is to make another movie with Audrey Hepburn," Although this sadly never happened, Grant was offered the role of Higgins in My Fair Lady (1964), and requested Hepburn as his co-star in Father Goose (1964)..
Cary Grant, who turned 59 during filming, decided it was time to stop playing the romantic lead after reviews focused on the 26-year age difference between him and Audrey Hepburn, who was 33 when the movie was made.
Seven studios rejected the original screenplay. Peter Stone turned it into a novel which was serialized in Redbook, and it was then turned back into a screenplay - which had interest from all 7 studios.
In the scene where Audrey Hepburn is smoking a cigarette alone in her empty apartment and Cary Grant enters, the backs of his ears had to be covered with masking tape since the backlighting made them appear red.
In the scene in which Audrey Hepburn spills ice cream on Cary Grant 's suit, she uses the term "assassination" and he uses the term "assassin." The movie was in release shortly after the Kennedy assassination in Dallas and Universal was so worried about audience reaction to this dialog that they hurriedly re-dubbed the lines, using other terms, then sent out a revised reel to every theater in America showing "Charade," telling them to substitute it for the old reel. Both old and revised reels may still be in circulation.
While much has been made about the age difference between the Cary Grant/Audrey Hepburn characters, if one does the math, her husband had some rank in the Second World War--nearly 20 years earlier. It is clear that Hepburn's character had a thing for older men.
In the dining boat scene, the background music is a vocal version of the theme song "Charade". It contains only three stanzas and the second stanza is completely different from the published lyrics. It reads: "And in a blaze of light For you Romeo came And it was closing night The ending of the play".
When at the end Audrey Hepburn runs a few metres from her hotel (near rue Monge) to get into a subway station, she actually arrives at Saint Jacques Station, a few kilometres from there. Then when she buys a ticket, she is even on a different line, miles away.
The trivia items below may give away important plot points.
The stamps depicted in the film are fictional counterparts of actual rare stamps, but have their values raised by one. The stamps they represent are the Swedish orange 3 skilling, the "Hawaiian Missionaries" 2 cent blue and the 81 para blue Romanian "cap de bour" on blue paper, in total worth about USA$3.6 Million in 2007.
Above the phone in Mrs. Lampert's room hangs a picture of the western side of Chateau du Chillon, a castle 3 km from Montreux, Switzerland. The castle itself isn't completely clear but the distinctive flat-topped range of snow-capped peaks in the background is unmistakable. Ironically, the castle itself has featured on several stamps.
Reggie asks Jean-Louis where he would hide a treasure, and he says he would hide it in the garden. Later it is discovered that the money had been used to buy rare stamps at the Jardin des Champs-Ëlysées - the Garden.