John le Carré was so impressed by Alec Guinness's performance as George Smiley that, in later novels, he wrote Smiley's characterization to be in keeping with Guinness' performance.
Alec Guinness was very concerned that he wasn't the right type to portray the "frog-like" George Smiley. Three weeks into filming, he panicked and asked to be replaced, and recommended Arthur Lowe for the role of Smiley. However, he eventually overcame his doubts and went on to receive critical acclaim for his portrayal.
John le Carré has admitted that the vocabulary used in the novels/series (babysitters, lamplighters, the Circus, the nursery, moles, scalp hunters) was made up. He was later amused to discover that real agents had begun to appropriate some of his vocabulary once his espionage stories were published.
The Czechoslovakia scenes were filmed in and around Glasgow, Scotland, which producer Jonathan Powell says "was a very rough, poverty-stricken place" in 1979.
Shortly before filming began, Alec Guinness asked author John le Carré to introduce him to a real spy to aid him in preparing for his role. Le Carré invited him to dinner with Sir Maurice Oldfield, who served as Chief of the British Intelligence Service from 1973 to 1978. During their meal, Guinness studied Sir Oldfield intently, noting any mannerisms and quirks he could use in his performance; when he saw Oldfield run his finger around the rim of his wineglass, and asked whether he was checking for poison, to Oldfield's astonishment, as he was only checking how clean the glass was.