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.
David Langton
In 1968 director Douglas Camfield chose Langton to portray Colonel Lethbridge-Stewart in the "Doctor Who" serial "The Web of Fear", but Langton dropped out to perform in a TV play before production began, so Camfield gave the part to Nicholas Courtney, who had originally been cast in different role. As played by Courtney, the character of Lethbridge-Stewart (better known as the Brigadier) returned to "Doctor Who" the next year and became one of its most recognisable supporting characters, appearing in "Doctor Who" irregularly until 1989 and making a final appearance in spin-off "The Sarah Jane Adventures" in 2008.