The cast has more than a few familiar faces. Sherlock star Martin Freeman will play Detective Superintendent Steve Fulcher, who'll be joined by Harry Potter alumni Imelda Staunton as Becky's mother Karen Edwards, and Siobhan Finneran, who's no stranger to British drama thanks to her memorable role in Happy Valley will play Sian's mother Elaine Pickford. Former Eastenders star Joe Absolom will play chief suspect Christopher Halliwell.