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.
Jon Pertwee
In 1969, Pertwee was selected by outgoing producer Peter Bryant and the series' next producer, Derrick Sherwin, to take over as the Doctor from Patrick Troughton in the television series "Doctor Who". Pertwee had asked his agent to apply for the role for him and was surprised to find he was already on the shortlist. He was the second choice for the role of the Third Doctor; Ron Moody was the first but was unavailable.