was once considered for the role of the Fourth Doctor. Hearne, however, wanted to play the Doctor as a version of Mr. Pastry, so he was passed over in favour of Tom Baker. The Doctor paraphrases philosopher Bertrand Russell when he advises his future self that "hate is always foolish and love is always wise". Production Cast notes Peter Capaldi reprised his role as the Twelfth Doctor. On 30 January 2017, he confirmed that the tenth series would be his last, and that he was set to leave after the 2017 Christmas special. The episode also sees the introduction of Jodie Whittaker as the Thirteenth Doctor, whose casting was announced on 16 July 2017. David Bradley appears in the episode as the First Doctor, having previously portrayed William Hartnell in the docudrama An Adventure in Space and Time. This makes him the third actor to play the role in the television programme, after William Hartnell and Richard Hurndall (in 1983's The Five Doctors) since the premiere of Doctor Who in 1963. Bradley previously played Solomon in the 2012 episode "Dinosaurs on a Spaceship". Since 2017, Bradley has reprised his role as the First Doctor in many audio dramas for Big Finish Productions alongside his co-stars from An Adventure in Space and Time. The first trailer for the episode was shown during the 2017 San Diego Comic-Con, revealing the return of Polly, a companion from the end of William Hartnell's tenure as the First Doctor, portrayed by Lily Travers, and Pearl Mackie as Bill Potts. Also confirmed were appearances by Toby Whithouse, writer of seven episodes between 2006 and 2017, and Mark Gatiss, also writer of nine episodes between 2005 and 2017, marking his fourth acting appearance in the series, playing a character credited as The Captain. In an interview following the episode's broadcast, Gatiss said he cried at the thought of playing the Brigadier's grandfather when he finished reading the script. However, the Haisman Literary Estate asserted their control of the backstory of the Brigadier, stating that Archibald was actually the Brigadier's great-uncle who appeared in Night of the Intelligence. They later compromised in What's Past is Prologue by implying that Archibald may secretly be the biological father of the Brigadier's father, thereby allowing for either interpretation. It was later confirmed that Ben Jackson, a companion of the First and Second Doctors, who served alongside Polly, would also feature in