Peter is the special guest actor in S4:E3 of modern Doctor Who, alongside David Tennant's Doctor. He of course also took that titular role himself, after Matt Smith.
There's a fun moment where the two grasp hands which is fun in hindsight.
Have other actors played both The Doctor and appeared in the show in other roles, either before or after their time as Doctor?
Colin Baker was cast as the Time Lord, Commander Maxil, in "Arc of Infinity" in 1983, and was the the 6th Doctor in 1984 to 1986.
David Bradley played the antagonist Solomon in Dinosaurs On A Spaceship (2012):
…before going on to play the First Doctor (as a replacement for the late William Hartnell) in The Doctor Falls, Twice Upon A Time (both 2017), and The Power Of The Doctor (2022):
He also voiced Azure Of The Claw Shansheeth in an episode of The Sarah Jane Adventures (Death Of The Doctor, 2010), and played William Hartnell himself in the docudrama An Adventure In Time And Space (2013).
(Several other actors have played the First Doctor too, most notably Richard Hurndall in the The Five Doctors in 1983. He was also portrayed as a child by Michael Jones in Listen, and he's been voiced by John Guilor, in archive footage shown in The Day Of The Doctor, and by Bradley, William Russell, and Stephen Noonan in audio dramas.)
As pointed out in a previous post, Colin Baker played a different role, before becoming the 6th Doctor.
Apart from this, it is quite common for a Doctor to meet his identical twin, played by the same actor. This can be the result of duplication, deliberate imposters, or just co-incidental resemblance.
1st Doctor - the Abbot in The Massacre.
2nd Doctor - Salamander in The Enemy Of The World
3rd Doctor - ?
4th Doctor - insane computer in The Face Of Evil, mini-clone in The Invisible Enemy, shape-shifting cactus in Meglos, army of duplicates in The Leisure Hive.
5th Doctor - Omega in Arc of Infinity, Kameleon in The King's Demons, android in Caves of Androzani
11th Doctor - Ganger in The Rebel Flesh/ The People, Teselecta in series 6 arc.
Treat that as a partial list, there's probably others.
The list of duplicates of companions is even longer. Of particular note is Jacqueline Hill, who played Barbara in the first few years of the show, and who later returned as Lexa in Meglos. As far as I know, this is the only time a series regular has returned in a different role. (As opposed to other actors who played different roles before becoming series regulars)
