Cast as Nathan Spring was David Calder, an experienced character actor with a reputation for "tough-guy" roles and best known at the time for his role as Detective Inspector George Resnick in the Lynda La Plante television serial "Widows". Naming him Nathan after his youngest son, Boucher had originally written Spring as a much younger character, a high-flyer in his early thirties who had risen rapidly through the ranks of the police. Despite this Boucher was pleased with Calder's performance describing Calder as "a class act" whose "performance was immaculate". Trevor Cooper was cast as Colin Devis at the suggestion of Graeme Harper who had worked with him on "Doctor Who" and in theatre. The rest of the cast, with the exception of Jonathan Adams (who had a semi-regular role on "Bergerac"), were relatively unknown.