Patrick McGoohan's daughter stood-in for Nadia Gray in the scene in which Number 6 and Number 8 pretend to get romantic. McGoohan refused to do romantic scenes with actresses due to his Catholic beliefs. Having his daughter in the scene prevented it from getting too romantic.
David Arlen is a native Londoner, and spoke no Russian before filming, and had to be coached for three days by someone from the Russian Embassy in London for the role. He also wasn't aware that Nadia Gray was a fluent Russian speaker, and criticised his grammar, teaching him the local dialect of the area Karel was supposed to come from.
The strange-looking propeller freight plane with the 747-like hump is an Aviation Traders ATL-98 Carvair.
During the art show, No 6 produces an abstract sculpture, and is asked by a puzzled competition judge, "There's just one thing I don't understand. Where is No 2?". Every other artwork in the competition is a depiction of the Village's leader. This can be seen as a parody of various twentieth century dictatorships, most notably those of Zedong Mao, Joseph Stalin and Adolf Hitler in which the supreme leader was sculpted and painted over and over again by various people, and had their portraits hung everywhere. Modern art is also sent up in The General (1967).
Nadia's interrogator after her attempt to escape by swimming is voiced by Robert Rietty, but briefly appears to be The Supervisor played by Peter Swanwick