DI Alec Hardy, played by David Tennant, complains to his doctor about doctors, "Always do as I say or you'll die." Tennant had previously played the role of Doctor Who (2005), a character who very much has that same attitude (deservedly).
Olivia Colman (Ellie Miller) & David Bradley (Jack Marshall) also worked together on Hot Fuzz (2007) as PC Doris Thatcher & Arthur Webley respectively.
Vicky McClure (Karen White) & Will Mellor (Steve Connelly) also worked together on two episodes of Line of Duty (2012) as Kate Fleming & Rod respectively.