In Series 2 Episode 2, DI Jack Armstrong (Toby Stephens) states the assumption that DI Georgina Dixon (Miranda Raison) was "watching endless episodes of 'Poirot'". Both actors played characters on Poirot; Toby Stephens as Philip Blake in "Five Little Pigs" (S9;E1) and Miranda Raison as Mademoiselle Blanche in "Cat Among the Pigeons" (S11;E2). DI Kate Bishop (Lucy Punch) did as well as Susannah Henderson in "After the Funeral" (S10;E3).
There was a gap of nearly two years between series 1 and 2. Series 1 concluded near the end of August 2010, while series 2 picked up in the beginning of August 2012. DI Bishop's departure during this time was never explained.