Actor Jack Lowden has discussed how his "Slow Horses" character River Cartwright is left deeply unsettled at the start of Apple TV+'s fifth season, following the previous season's revelation that his father, Frank Harkness (Hugo Weaving), is a sociopathic assassin who fathered and trained numerous sons as killers. Lowden explains that River's damaged mental state carries through the new season, making his usual MI5 posting at Slough House, under Gary Oldman's Jackson Lamb, feel increasingly unbearable, though the character does move towards a more hopeful place by season six.

Lowden says he drew on thoughts of his own grandfather while filming, noting that River is driven by pride and a determination to make his grandfather proud, even as his personal life unravels. He points to River's grandfather David, played by Jonathan Pryce and struggling with dementia, as a key emotional anchor, with tender "garden scene" exchanges between the two offering glimpses of hope amid the grief. Lowden also touches on a plot thread involving River lying to Lamb about the death of politician Dennis Gimball in the episode "Circus", framing it as a product of River's petty but very human pride.