There is a letter in the Hughes Massie archive at Exeter University from Agatha Christie to her agent, requesting that he ask for legal advice on what would happen should Person A's alibi appear a year after Person A had been found guilty and died in prison.
Matthew Goode said Philip was "quite a fun character to play, because there's just no limit to the nastiness that you can bring to him, or the duplicity." Matthew said that he actually apologized to some of his cast mates after some of the takes.
Dr. Arthur Calgary, played by Luke Treadaway, was shot using different lenses and different frame speeds than other characters to create a sense of otherness.
Matthew Goode's character, Philip Durrant, doesn't wear a wedding ring, because Matthew thought his character would have pawned everything of value. This is also why you won't see him wearing a watch.
Ella Purnell said that Audrey Hepburn was an inspiration for her character, Hester.