Frazer Hines
Hines' "Doctor Who" debut came in 1966, when he was cast to play the part of Jamie McCrimmon, the companion of the Second Doctor (played by Patrick Troughton). Originally intended as a one-off guest character, Jamie joined the regular cast and appeared in the series from 1966 to 1969. Hines reprised the role in a cameo in the 20th anniversary serial "The Five Doctors" (1983) and as a guest star in "The Two Doctors" (1985). Hines would appear in 117 episodes of "Doctor Who" in all—more than any other "companion" actor in the history of the series. The only actors appearing in more episodes are those who played the first four Doctors. Many of the episodes featuring Jamie no longer exist in the BBC's collection.
Midnight (Doctor Who)
David Troughton, cast here as Professor Hobbes, was a late replacement for Sam Kelly, who broke his leg and had to withdraw from the production. Troughton joined the rest of the cast in Cardiff with just two days notice. Now known for his stage work with the RSC as well as television, he is the son of Patrick Troughton, who portrayed the Second Doctor. He has had long association with the series since the 1960s, appearing as an uncredited extra in the Second Doctor serial "The Enemy of the World", as Private Moor in the Second Doctor serial "The War Games", and as King Peladon in the Third Doctor serial "The Curse of Peladon". He has appeared in the "Doctor Who" audio dramas "Cuddlesome", "The Judgement of Isskar", "The Destroyer of Delights", and "The Chaos Pool". He is unrelated to the director of the episode, Alice Troughton.