 John Bell was a nine-year-old who won a "Blue Peter" competition to appear in this episode.John Simm reprised his role as the Master in the Tenth Doctor's final story "The End of Time" and reprised the role again in "World Enough and Time" and "The Doctor Falls", the finale of Series 10, the final series to feature Peter Capaldi as the Twelfth Doctor and Michelle Gomez as Missy.Simm will reprise the role once again in the upcoming audio drama "Masterful" by Big Finish Productions.