Michael Caine's Harry Palmer character 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 the movie is based. Deighton himself 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 (Michael Caine is myopic in real life). Caine chose to wear glasses because he expected the film to be the first of a series, 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.
Harry's glasses frames were dark brown, contrary to the widely held view that they were black. They were a style called "Teviot 74" manufactured by a company called UK Optical. They were already popular at the time for being a stylish and inexpensive alternative to the standard models that were issued for free by the National Health Service in Britain. They became even more popular after the success of this film. Len Deighton wore the same frames at this time. Those frames have been described by some as the first affordable "designer" frames available in the UK.
Two large Victorian terrace houses, at 28 and 30 Grosvenor Gardens, London, were used as studios. The two houses were converted into one huge house containing 40 rooms. These were enlarged or divided according to requirements. Fourteen rooms were used as studios. Other rooms were turned into dressing rooms, wardrobe department, hairdressing, make-up, production offices, a property department and a self-contained restaurant capable of feeding and seating 120 people! This all was kept secret to keep away sight-seers and autograph hunters. Even Michael Caine was driven to work in an inconspicuous car and had to sneak in the back way. As a front, a large sign was painted at the entrance to the film studios. The sign read "The Dalby Employment Agency".
In the Len Deighton novels the name of the lead character is never revealed. So Michael Caine and producer Harry Saltzman tried to think of a boring name for the hero. Caine suggested "Harry" which Saltzman found rather amusing. Caine then remembered a boring classmate named Tommy Palmer. So "Palmer" became the surname.
Although the narrator in the novel is nameless, at one point in the novel he is greeted by someone saying "Hello, Harry". The narrator then thinks "Now my name isn't Harry, but in this business it's hard to remember whether it ever had been."
Three pairs of glasses were used by 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 20 extra pairs of the Harry Palmer model glasses.
This movie proved to be a major influence on the style and ambiance of the popular TV series, Mission: Impossible (1966). TV producer/director Bernard L. Kowalski had seen "The Ipcress FIle" and was so impressed he requested that a similar mood and urgency be emulated for "M:I."
In the years following the film's release, Harry Saltzman claimed that he had fired Sidney J. Furie relatively early in shooting, and that Peter R. Hunt had really directed most of the film, with Furie only being credited as director for contractual reasons. Hunt denied this, however, and revealed that he had in fact tried to preserve Furie's original vision to the best of his abilities, despite Saltzman's attempts to do otherwise.
Producer Harry Saltzman hated director Sidney J. Furie and his oddball style and went so far as to bar him from the editing room. According to Furie, Saltzman also excluded him from the film's party at Cannes and even stole his best picture British Academy Award.
Twice Dalby is seen to have an anomalous shadow: once on the projector screen after the projector has been turned off and again in the warehouse when he is asked to stand under the light yet he casts a strong shadow against the wall. Assuming these are not goofs they are surely meant as metaphors for his shadowy nature.
At the start of the film, the station (Marylebone) announcer says that the (steam-) train is leaving for 'Nottingham' and 'Leicester' which meant that it would have had to travel along the former "Great Central" route towards Sheffield: this line, one of the most modern main-lines in the country (from 1899), was closed the following year  and most of the trackbed has been built-over or erased completely from the map.