Nicola Jane Walker is an English actress, known for her starring roles in various British television programs from the 1990s onward, including that of Ruth Evershed in the spy drama Spooks from 2003 to 2011 and DCI Cassie Stuart in Unforgotten from 2015 to 2021. She has also worked in theatre, radio and film. She won the 2013 Olivier Award for Best Supporting Actress for the play The Curious Incident of the Dog in the Night-Time, and has twice been nominated for the BAFTA TV Award for Best Supporting Actress for the BBC drama Last Tango in Halifax.