- California Doubling: The Czechoslovakia scenes were filmed in and around Glasgow.
- Creator-Preferred Adaptation: John le Carré was so impressed by Alec Guinness' performance as George Smiley that in later novels he wrote Smiley's characterization to be in keeping with Guinness's performance.
- Playing Against Type: This was a rare dramatic role for Beryl Reid, who was known for light comedy. Alec Guinness was initially opposed to her casting, but she won him over.
- Troubled Production: The series was plagued by a series of strikes. Due to shooting overruns, a large number of actors moved to other projects without finishing their roles.
- Actor Allusion: It seems that John Hurt always had a lot of experience with British political extremes and secret services.
- All-Star Cast
- Awesome, Dear Boy: Benedict Cumberbatch stated that playing a spy was his dream job - "you are NEVER what you seem."
- Dawson Casting: Karla's daughter is supposed to be in her twenties but the actress is in her thirties.
- Dyeing for Your Art:
- Fan Nickname: Due to being directed by Tomas Alfredson, this film gained the nickname of "Get the Wrong One Out"
- In Memoriam: The film is dedicated to screenwriter Bridget O'Connor, who completed work on the film before her passing away from cancer.
- Playing Against Type: Colin Firth and Mark Strong co-star in a movie where one plays a kindly schoolteacher haunted by his past, and one plays a depraved turncoat spy selling international secrets to the Soviets. Sounds pretty typical, unless you know that Strong's the schoolteacher and Firth's the evil spy.
- What Could Have Been:
- Oldboy director Park Chan Wook was offered the film at one point. In 2017 he eventually tapped to direct a six-part miniseries adaptation of John le Carré's The Little Drummer Girl.
- Michael Fassbender was originally cast as Ricki Tarr, but he had to back out because he was busy filming X-Men: First Class.
- David Thewlis and Ralph Fiennes were considered for roles.