George Smiley: What did you think of Control's theory?
Jim Prideaux: I thought it was madness!
At the start of the movie, Control, played by John Hurt, signs a letter with a green pen. This is based on reality - traditionally the head of the Secret Intelligence Services or MI6 always uses a green pen to sign letters with a C. See more...
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Factual error: George Smiley stays in a hotel near Liverpool St Station, London to investigate the spy at the heart of the circus. There is one scene where we see, from above, a train leaving the station. Liverpool St station is electrified using overhead wires - the train is leaving a station using an different electrification system called 3rd rail which has never existed at Liverpool St. I'm also pretty sure, from the roof pattern of the train that is a type introduced from the early 1980s onward.