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.
Second Doctor
Due to what would appear to be continuity errors in Troughton's later appearances (particularly in "The Two Doctors"), some fans have speculated that the Time Lords used the Second Doctor as an agent after the events of "The War Games" (1969), and that he did not in fact immediately regenerate and enter his exile on Earth. This theory of continuity is described as "Season 6B". Additionally, while "The War Games" never depicted the Doctor's regeneration itself, it was later depicted in "TV Comic", which treated the exiled Doctor's exploits on Earth, before, in a comic entitled "The Nightwalkers", showing the Doctor dragged in to the TARDIS by animated scarecrows, allowing his change of appearance to take place. The Second Doctor's regeneration was also depicted in "Devious", a fan-produced film starring Jon Pertwee in his last appearance as the Third Doctor.