Sharon Small (Nora Harding, episode 2x05) previously played DS Barbara Havers on The Inspector Lynley Mysteries. Jenny Agutter (Sister Julienne) made a guest appearance in episode 3x03 of that programme, A Cry for Justice, playing Jemma Sanderson.
Real-Life Relative: Screenwriter/show creator Heidi Thomas is married to Stephen McGann, who plays Dr Turner. (McGann good-naturedly grumped that just because he's married to the head honcho doesn't mean he gets a preview of what's coming; he had to wait to find out whether Sister Bernadette and his character were going to end up together until he got his script. For the record, they did.)
Real Life Writes the Plot: Episode 3x01 dealt with cystic fibrosis because Jenny Agutter lost two infant siblings to the disease, and her niece has it as well; she has since focused most of her charity work on finding a cure.
Shipper on Deck: Stephen McGann and Laura Main ship their characters almost as desperately as the fans do. Possibly more.