HMP Blackthorn, where Michael Farmer is on remand, is the same prison former Chief Superintendent Fairbanks was sentenced to in Series 3.
GPD is short for General Purpose Dog.
Simon Millar has played four different characters over the course of the series:
- Episode 4.5 Lying Nest (2017) - DC Daniel Brightway (uncredited)
- Episode 4.4 Moral Superiority (2017) - DC Daniel Brightway (uncredited)
- Episode 4.3 In the Trap (2017) - DC Daniel Brightway
- Episode 4.2 Who Sows the Wind (2017) - DC Daniel Brightway
- Episode 4.1 In the Shadow of the Truth (2017) - DC Daniel Brightway (uncredited)
- Episode 3.3 Snake Pit (2016) - Cameraman (uncredited)
- Episode 2.2 Carly (2014) - Passer By (uncredited)
- Episode 2.1 The Ambush (2014) - Mourner (uncredited)
Lee Ingleby (Nick Huntley) & Jason Watkins (Tim Ifield) also worked together on Watership Down (2018) as the voices of Captain Campion & Captain Orchis respectively.