Harry Palmer is depicted as an accomplished cook, but when you see Palmer skillfully break a couple of eggs, the hands in the close-up belong to Len Deighton, author of the book on which this movie was based. Deighton was an accomplished cook and also wrote a comic strip about cooking for The Observer. The walls of Palmer's kitchen are full of these strips.
Palmer is the first action hero to wear glasses. (Sir Michael Caine is near-sighted in real life.) Caine chose to wear glasses because he expected this movie to be the first of a franchise, similar to the Bond movies. He feared being over-identified with the character of Harry Palmer, and so he wore the glasses so that he could remove them for other roles.
On the first day of shooting, director Sidney J. Furie gathered the cast and said, "This is what I think of the script." He then set it on fire.
The main melody in the score was played by John Leach on a cimbalom, a type of Hungarian dulcimer, that provided the forlorn mood that composer John Barry was eager to create.
Three pairs of glasses were used by Sir Michael Caine during filming. When all of these were broken during filming, production was held up for a day, until replacements had been found. After that, the Prop Department was stocked with twenty extra pairs of the Harry Palmer model glasses.