Jack Hawkins and Charles Gray appear together. After Hawkins lost his voice due to throat cancer in 1966, Gray was one of the actors used to dub his voice in later films.
This film was originally planned as a vehicle for Rex Harrison. After some false starts, it was eventually decided that Jack Hawkins should have an American co-star to play the role of "Fraser"; this necessitated extensive script re-writing. Cliff Robertson agreed to play the part, but insisted on an American writer to do the rewrite. He called in the novelist William Goldman, whom he had previously hired to write a film script based on the science-fiction novel "Flowers for Algernon", by Daniel Keyes. Goldman got his first film writing credit on "Masquerade", but when Robertson finally got to make the "Flowers for Algernon" film in 1968 (renamed "Charly", another screenplay, by Stirling Silliphant, was used.
At one point early in the film, Jack Hawkins says he envies Charles Gray his age "being 10 years younger." In point of fact, Gray was 18 years younger than Hawkins.
This was the first of several English-language roles for French actor Michel Piccoli.
A scene filmed at London Airport had such a lot of background noise that Ernest Clark's dialogue had to be dubbed at the studio but the film's director wasn't available so it was attended to by a sound engineer. The director wasn't satisfied and with Clark also not available it was done by someone else.