After more than 60 actresses had been auditioned, the first choice to play the role was Elizabeth Shepherd. However, after filming one and a half episodes (the pilot, "The Town of No Return", and part of "The Murder Market"), Shepherd was released. Her on-screen personality was deemed less interesting than that of Blackman's Gale, and it was decided that she was not right for the role. Another 20 actresses were auditioned before the show's casting director, Dodo Watts, suggested that producers Brian Clemens and Albert Fennell check out a televised drama featuring the relatively unknown Rigg (she had earlier guested in an episode of "The Sentimental Agent" that Clemens had written). Rigg's screen test with Macnee showed that the two immediately worked well together.