Century 21 had first experimented with extensive live-action location shooting during the production of Thunderbird 6 (1968). "The Unorthodox Shepherd" had also been filmed partly on location. Commentator Stephen La Rivière notes the contrast between the format of The Secret Service and those of earlier Supermarionation series, whose live-action elements were usually limited to occasional shots of human hands performing actions too complex for the puppets, such as operating machinery. The only new puppets created for The Secret Service were those of Unwin, his housekeeper Mrs Appleby (which sculptor Christine Glanville based on her mother) and The Bishop. Supporting characters were played by puppets recycled from Joe 90 and Captain Scarlet; the Matthew Harding puppet was originally made for the Captain Scarlet episode "Treble Cross" while Blake was played by the Captain Scarlet puppet himself. La Rivière describes the puppet Unwin as "one of the most impressive artistic feats" to be produced by the Century 21 puppet workshop, calling the likeness "uncanny". It is possible to directly compare the puppet with the real-life Unwin in every episode as the opening credits show the real Unwin's face, rather than the puppet's. Voice cast With the exception of Unwin, all voice actors on The Secret Service had supplied voices for earlier Supermarionation series. The Secret Service featured the voices of: Stanley Unwin as Father Stanley Unwin (aged 57), a parish priest in rural England who carries out top-secret missions for BISHOP (British Intelligence Service Headquarters, Operation Priest). Due to the realistic vicar costume that he wore on location, passing members of the public often mistook Unwin for a real vicar. Gary Files as Matthew Harding (aged 28), Unwin's partner in BISHOP as well as gardener of his vicarage. Files fondly remembered his time on The Secret Service, stating that he enjoyed voicing Harding more than Captain Magenta in Captain Scarlet. He made significant contributions to Harding's characterisation, devising the character's rural accent himself: "It just seemed so right for the character. Then once I had the voice, the rest of Matthew followed." Sylvia Anderson as Mrs Appleby (aged 55), Unwin's housekeeper, who is kept ignorant of Unwin and Harding's double life. Jeremy Wilkin as The Bishop (aged 52), a high-ranking British Intelligence official and head of BISHOP, based in Whitehall. Keith Alexander as Agent Blake, a cautious junior agent of BISHOP, who has speaking roles in three episodes ("A Question of Miracles",