 Writer Steven Moffat said that the dual-role required a very skilled actor, and a well-known actor was necessary because it was such an expensive show to produce.The production team decided Nesbitt's two characters would be mainly differentiated over a change in performance rather than by extensive make-up because they wanted Hyde to be able to walk around in public without attracting attention.Michelle Ryan, known for her long-running role as Zoe Slater in the soap opera "EastEnders", was revealed by tabloid newspaper "The Sun" to have been cast as "Jekyll's sultry assistant" (Katherine Reimer) in August 2006.