Thomas Chelimsky was dubbed by a French woman.

Cary Grant initially turned down the film, after which it was briefly considered a possible vehicle for Warren Beatty and Natalie Wood.

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.

Cary Grant, who celebrated his 59th birthday 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 only 33 when the movie was made.

According to Audrey Hepburn, the scene where Regina spills ice cream on Adam's suit is based on a real-life accident where Hepburn spilled red wine over Cary Grant's suit at a dinner party.

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, and requested Hepburn as his co-star in Father Goose..

Charles Lampert's passports found in the evidence bag are from the following countries, in order: Switzerland, the United States, Chile and Italy.

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".

Filmed virtually back-to-back with Paris - When It Sizzles. The small park with its Punch and Judy puppet stage appears in both films.

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".

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.

It was agreed that Cary Grant would not remove his shirt in the shower scene since he was nearly sixty and slightly overweight.

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.

The character of Peter Joshua was named after Stanley Donen's two sons: Peter and Joshua.

The character played by Cary Grant quotes a line from My Fair Lady ("On the street where you live"), the film version of which would star Audrey Hepburn the following year.

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.

The voice of the Marine in front of the Embassy is dubbed by screenwriter Peter Stone.

The young man in the Embassy elevator telling the story about the poker game is screenwriter Peter Stone - with the dubbed voice of director Stanley Donen.

This film is public domain due to the failure to put the then-required copyright notice in the released print. The supposed copyright notice in the film failed to include the text "Copyright", "Copr." or "©", as was needed by pre-1989 US law (only the year and supposed copyright holder were listed).

While much is/has been made about the age difference between the Cary Grant/Audrey Hepburn characters, If one does the math, (her husband - who never appears in the film - had some rank in the Second World War - nearly 20 years earlier), and looks at the characters who played her husband's contemporaries, it is clear that Hepburn's character had a thing for older men.