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.
Doctor Who and the Daleks in the Seven Keys to Doomsday
Originally, the part of the Doctor was to be played by Jon Pertwee, but the role went instead to character actor Trevor Martin. Martin had previously appeared in "Doctor Who" as a Time Lord in the 1969 serial, "The War Games". In the play, it is stated that Martin's version of the Doctor is regenerated from Pertwee's; this makes him an alternate version of the Fourth Doctor. (The play premiered about two weeks before Tom Baker began his television tenure as the Fourth Doctor).