The source novel "The Berlin Memorandum" is billed in the credits as being by Adam Hall. This is a nom de plume for author Elleston Trevor.
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.
Charlton Heston was courted to play Quiller, and was keen to work with a Harold Pinter script, but it didn't work out.
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)."