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.
Sylvester McCoy
McCoy became the Seventh Doctor after taking over the lead role in "Doctor Who" in 1987 from Colin Baker. He remained on the series until it ended in 1989, ending with "Survival" (see List of Doctor Who episodes (1963–1989)). As Baker declined the invitation to film the regeneration scene, McCoy briefly wore a wig and appeared, face-down until the last moment before the regeneration commenced, as the sixth Doctor. He played the Doctor in the 1993 charity special "Dimensions in Time", and again in 1996, appearing in the beginning of the "Doctor Who" television movie starring Paul McGann as the Eighth Doctor.