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 Tennant
Tennant featured as the Doctor in an animated version of "Doctor Who" for "Totally Doctor Who", "The Infinite Quest", which aired on CBBC. He also starred as the Doctor in another animated six-part "Doctor Who" series, "Dreamland". Tennant guest starred as the Doctor in a two-part story in "Doctor Who" spin-off "The Sarah Jane Adventures", broadcast in October 2009. He continued to play the Tenth Doctor into the revived programme's fourth series in 2008. However, on 29 October 2008, he announced that he would be stepping down from the role after three full series. He played the Doctor in four special episodes in 2009, before his final episode aired on 1 January 2010, where he was replaced by the eleventh doctor, portrayed by Matt Smith. The "Daily Mirror" reported that Tennant was forbidden from attending "Doctor Who" fan conventions while playing the role, to avoid the chance that he could accidentally let slip any plot points during filming of the series. However, Tennant claimed this was false and that he had never been banned or discouraged from attending conventions.