The source novel "The Berlin Memorandum" is billed in the credits as being by Adam Hall. This is a nom de plume for author Trevor Dudley Smith.

Even though this movie was made over twenty years after the end of World War II, the devastation of Berlin was so vast, the set decorators weren't required to "re-create" exterior areas for filming, just interiors.

In adapting (Trevor Dudley Smith writing under the nom de guerre of) Adam Hall's novel "The Berlin Memorandum", Screenwriter Harold Pinter altered the emphasis of the book to be less a spy thriller and more a meditation on the human condition, and the duplicitous nature of identity.

The score was composed by John Barry, who became notable for the early James Bond spy movie soundtracks. Barry composed the score for this movie between Thunderball (1965) and You Only Live Twice (1967).

George Segal is the only actor to play the character Quiller on film.

BBC produced a television series, Quiller (1975), about the Quiller character.

The theme music played throughout this movie was recorded by Matt Monro as "Wednesday's Child". The song is playing on the radio in the reception of Quiller's hotel, and appears on several of Monro's albums.

Charlton Heston was courted to play Quiller, and was keen to work with a Harold Pinter script, but it didn't work out.

The source novel "The Berlin Memorandum" won the Edgar Allan Poe Award for Best Novel in 1966. The book was also awarded the Grand Prix de Littérature Policière.

Sir Alec Guinness appeared in this espionage movie thirteen years before he played John le Carré's famous spy character George Smiley in Tinker Tailor Soldier Spy (1979).

This movie received three BAFTA nominations: Best British Screenplay, Best British Art Direction (Colour), and Best British Film Editing, but failed to win any.

One of four spy movies that Senta Berger appeared in, in 1966. The others being Bang! Bang! You're Dead! (1966), The Poppy Is Also a Flower (1966), and The Ambushers (1967).

After Operation Crossbow (1965), this was Director Michael Anderson's second consecutive espionage movie.

The name of the intelligence agency that Quiller (George Segal) worked for was MI6.

The movie's themes song, "Wednesday's Child", can be heard playing on a radio when Quiller enters his hotel (with the Phoenix members outside).

George Sanders and Robert Helpmann received "guest star" credits.

Max von Sydow (Oktober) portrayed the famous spy movie villain character of Ernst Stavro Blofeld in the unofficial James Bond movie, Never Say Never Again (1983).

This movie was released a year after the source novel "The Berlin Memorandum" by Trevor Dudley Smith (as Adam Hall) was published.

The telephone number Quiller tells Inga to remember is 218962.

Alec Guinness and Max von Sydow appeared in films of the Star Wars franchise. Guinness starred in Star Wars: Episode IV - A New Hope (1977), Star Wars: Episode V - The Empire Strikes Back (1980) and Star Wars: Episode VI - Return of the Jedi (1983) while von Sydow was in Star Wars: Episode VII - The Force Awakens (2015).

The movie's major cast feature two Georges, George Segal and George Sanders, as well as two Roberts, Robert Flemyng and Sir Robert Helpmann.

Michael Jayston is the second (and as of 2017) last actor to play the character, Quiller, having done so on the television series Quiller (1975).

The film cast includes two Oscar winners: George Sanders and Alec Guinness; and two Oscar nominees: George Segal and Max von Sydow.