As well as the series debut coinciding with the revelation that Sir Anthony Blunt had been a member of the Cambridge Five spy ring, Ian Richardson (Bill Haydon) also went on to play Blunt in a 1986 British television production (which also starred Anthony Hopkins as Guy Burgess).
One of two appearances in two filmed John le Carré adaptations featuring actor Joss Ackland. The first was in this episode and the second and final appearance was in the tele-movie A Murder of Quality (1991). Ackland appeared in each type of television adaptation of a John le Carré, both a tele-movie and a TV mini-series. The two TV productions were made and first broadcast around a dozen years apart.
When Smiley is dining with Jerry Westerby (Joss Ackland), the two bottles of beer on the table are Löwenbräu. The bottles are green, light blue labels on the front and back, and silver foil on the neck.
Jim Predeaux (Ian Bannon) refers to a watcher as a 'juju man'. According to Encyclopeadia Brittanica, the phrase is generally of West African origin, and can refer to magic or magical power. In Predeaux's context, its use as a phrase employed in the "Circus", connotes dark magic, evil spirit, or a boogieman.