was asked to helm its replacement due to his experience, and so Fiona Cumming was asked to take over Enlightenment. Once production began it became apparent that Part One and Part Two were under-running so more dialogue was written to fill in the time. It was originally intended that the character of Jackson would not reappear after the second episode, but during filming Saward became concerned that it appeared that he had been executed and so he and Clegg rewrote Part Three to include him. Part Three also looked to be under running so scenes from Part Four were brought forward and the final scenes with the Guardians were extended to compensate. Casting Cumming came up with the idea that the Eternals would not blink and cast actors who she believed could provide detached performances. Cumming recalled Peter Sallis had played a similarly detached character in the 1974 BBC drama The Pallisers, and she cast him in the role of Striker. Sallis was present during the rehearsals for the serial but when production was delayed he was unavailable for the new filming dates, being committed to filming Last of the Summer Wine, and was forced to drop out of the production, being replaced by Keith Barron. Lynda Baron was cast as Captain Wrack, having previously participated in Doctor Who in the 1966 serial The Gunfighters as the voice that sings the "Ballad of the Last Chance Saloon" heard throughout that story. Tony Caunter, who had previously played Thatcher in The Crusade and Morgan in Colony in Space, was given the role of Jackson, with Christopher Brown and David Rhule being cast as Marriner and Wrack's sidekick Mansell respectively. Similarly to Sallis, David Rhule was unavailable for the revised filming dates in January, so singer Leee John replaced him at short notice, despite having no previous acting experience. Valentine Dyall had originally played the Black Guardian in the 1979 serial The Armageddon Factor, the character's first appearance in the series, and reprised the role for season 20, appearing in Mawdryn Undead, Terminus and Enlightenment. Similarly, Cyril Luckham reprised the role of the White Guardian that he had previously played in the 1979 serial The Ribos Operation. Design The interior sets of the boats were not built specifically for the programme, but were pulled together from stock items from various prop warehouses. Cumming had originally hoped to simulate the rocking of the