Holly Aird (Dr. Wharton) was expecting her second child during season 4 which is why in season 4 her character is nearly always seen standing behind something (or someone in walking shoots) or wearing large coats and lab coats.
In 2018, a five-part radio prequel to the series, The Unforgiven, was broadcast on BBC Radio 4, with Sue Johnston, Claire Goose, Wil Johnson and Holly Aird reprising their roles as Dr. Grace Foley, DS Amelia 'Mel' Silver, DI Spencer Jordan & Dr. Frankie Wharton respectively. All five episodes were written by series creator Barbara Machin.
The trivia item below may give away important plot points.
When Claire Goose (Amelia) left Casualty (1986) she originally asked to be killed off, but the producers refused. When she began working on this series, the same producers also began working on the show. When she decided to leave at the end of S4 she told the producers they would have to kill her off now.