Director Cameo: [Alfred Hitchcock] early in the film sitting in a hotel lobby with a baby on his knee.
The scene where Gromek is killed was written to show how difficult it really can be to kill a man.
Keith Waterhouse and Willis Hall did extensive (uncredited) rewrites on the script.
'Herrmann, Bernard' wrote the original score, but Universal Pictures executives convinced Hitchcock that they needed a more upbeat score. Hitchcock and Herrmann had a major disagreement, the score was dropped and they never worked together again.
The Swedish actor Jan Malmsjö (who had a small uncredited role as photographer in the final scenes in Helsingborg harbour and customs) found that a lot of signs were not written in correct Swedish so he helped the film crew to correct them.
In the shot in which Alfred Hitchcock's cameo occurs, the music briefly changes to "Funeral March of a Marionette" by Charles Gounod, which is best known as the main theme for "Alfred Hitchcock Presents" (1955).
[first lines]
Professor Karl Manfred:
Are they ever going to get the heating fixed?
Norwegian crewman:
They are working at it, Professor. Perhaps some of you scientists would like to give us a helping hand!
[last lines]
Dr. Sarah Louise Sherman:
Oh, Michael!
[laughing]
Factual errors: A few snippets of dialogue in the scenes in the East German university clearly show that the extras are Americans.
Boom mike visible: Reflected in the window of the farmhouse as Armstrong enters.
Crew or equipment visible: Or their shadow, anyway. On the road-level shot of Armstrong's taxi leaving the farm (Gromek's motorbike is visible on the left of the screen), just at the very bottom of the image can be seen the shadow of the camera (4:3 television version only).
Revealing mistakes: In the farmhouse scene when Professor Armstrong is fighting with Gromek, Gromek is choking Professor Armstrong but Armstrong shows no signs of it, i.e. bruising, in later scenes.
Continuity: When Armstrong and Lindt are in Lindt's workroom, Lindt erases only the right half of the chalkboard. However, when Armstrong starts writing, the entire board has been erased.
Referenced in Plotting 'Family Plot' (2001) (V)