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
There have been instances of actors returning at later dates to reprise the role of their specific Doctor. In 1973's "The Three Doctors", William Hartnell and Patrick Troughton returned alongside Jon Pertwee. For 1983's "The Five Doctors", Troughton and Pertwee returned to star with Peter Davison, and Tom Baker appeared in previously unseen footage from the uncompleted "Shada" episode. For this episode, Richard Hurndall replaced William Hartnell. Patrick Troughton again returned in 1985's "The Two Doctors" with Colin Baker. In 2007, Peter Davison returned in the Children in Need short "Time Crash" alongside David Tennant, and in 2013's 50th anniversary special episode, "The Day of the Doctor", David Tennant's Tenth Doctor appeared alongside Matt Smith as the Eleventh Doctor and John Hurt as the War Doctor, as well as brief footage from all of the previous actors. In 2017, the First Doctor (this time portrayed by David Bradley) returned alongside Peter Capaldi in "The Doctor Falls" and "Twice Upon a Time" (both 2017). In "The Name of the Doctor" (2013), the Eleventh Doctor meets a previously unseen incarnation of himself, subsequently revealed to be the War Doctor. Additionally, multiple incarnations of the Doctor have met in various audio dramas and novels based on the television show.