This is the second time Robin Bailey plays a role previously played by Arthur Lowe (Lowe played "Charters" in the 1979 version of La dama del expreso (1979)) The first was the titular role of Potter (1979) in the final series after Lowe had died.
Charters and Caldicott were original characters created by Sidney Gilliat and Frank Launder for their Alarma en el expreso (1938) screenplay, the characters were not in Ethel Lina White's book The Wheel Spins (1936). Gilliat and Launder next added the characters to their script for Tren nocturno a Munich (1940). These comic relief side characters proved so popular that Gilliat and Launder made them the leads in a radio serial that was later adapted as the film Crook's Tour (1940). The characters also appeared in a further radio serial and the film Millones como nosotros (1943). They were portrayed by Basil Radford and Naunton Wayne until they had a dispute over the size of the roles they were to play in Gilliat and Launder's I See a Dark Stranger (1946). The actors left the film but continued to play similar comic duos in film and radio but for copyright reasons were never again named Charters and Caldicott. Gilliat and Launder's screenplay was the basis for La dama del expreso (1979) in which the characters were portrayed by Arthur Lowe and Ian Carmichael and the setting remained the eve of WWII. The characters are in contemporary times for this series.