We start with probably the most famous 'other' Doctor, or indeed Dr to give him his actual title. Peter Cushing played Dr. Who in the two 1960s movies Dr. Who & the Daleks and Daleks' Invasion Earth 2150AD. No longer from Gallifrey, this Dr was a human. He was a scientist who built his own time-travel machine and named it TARDIS. But Dr. Who was clearly an alternate version of the First Doctor played by William Hartnell, we can be sure of that because regeneration hadn't been thought of when the first movie debuted.
During the First Doctor's era several other uncredited actors stood in for Hartnell:
During restoration work for the Planet Of The Giants DVD release, John Guilor was bought in to re-dub some of Hartnell's voice for episodes 3 and 4. He was then called upon to provide the First Doctor's vocal cameo in The Day Of The Doctor.
One more television First Doctor for you. After Hartnell passed away in 1975 Richard Hurndall portrayed the "original" in the 20th anniversary story The Five Doctors.
Patrick Troughton had two doubles during his time as the Second Doctor:
As well as appearing in The Curse Of Peladon in 1972 and Midnight in 2008, Patrick Troughton's son, David Troughton, played a clone of his Father's Doctor in the 2011 BBC Audiobook Serpent Crest.
In 1999 Mark Gatiss played a version of the Doctor in the spoof sketch The Web Of Cave. Although it's unclear which Doctor he is supposed to be playing, the style of the sketch allows us to assume it was either the First or Second.
It's worth also mentioning An Adventure In Space And Time, although it was a dramatic recreation of Doctor Who's journey to the screen during production many of the original classic scenes from the 60s were recreated with new actors. David Bradley played the First Doctor, Reece Shearsmith was the Second Doctor and Mark Gatiss portrayed the Third Doctor, although his scenes all ended up on the cutting room floor.
On stage Nick Scovell played both the First and Second Doctors in productions of The Dalek Masterplan, The Web Of Fear, Fury From The Deep and Evil Of The Daleks. During 'Masterplan' his Doctor regenerated into long serving Doctor Who vocal actor, Nicholas Briggs.
For the Big Finish 50th anniversary special The Light At The End, former Who companions provided the voices for their respective Doctors, William Russell played the First Doctor and Frazier Hines was the Second Doctor. Both did excellent jobs. Tim Treloar provided the voice of the Third Doctor for this release. You can read more about that here.
After appearing as the Cyberleader in several 1980s episodes of Doctor Who, David Banks (above centre) joined the cast of the Doctor Who stage show The Ultimate Adventure. He played Karl the Mercenary and also acted as Jon Pertwee's understudy. When Pertwee was taken ill, Banks stood in as the Doctor for two performances.
Although Tom Baker had been seen briefly at the end of Planet Of The Spiders, as Jon Pertwee regenerated, another actor would play the Fourth Doctor before Baker properly took control of the Tardis. Trevor Martin had already played a Time Lord in the Second Doctor adventure The War Games, then in 1974 he portrayed the Fourth Doctor on stage at the Adelphi Theatre in the Terrance Dicks penned Seven Keys to Doomsday. The play debuted two weeks before Baker's debut story, Robot.
The play came back to the stage in 1984, this time in New Zealand where Michael Sagar played the Doctor. Although he is never referred to as the Fourth Doctor during this version, it is assumed that he is the same incarnation as the original production. Trevor Martin then returned to the role for the Big Finish adaptation in 2008. Interestingly Nicholas Briggs had a cameo as the Third Doctor in this Big Finish production, and as I mentioned above he'd already played this role in the theatre so it gave some kind of continuity to the stage adventures.
Nicholas Briggs also portrayed another version of the Doctor in the Big Finish Doctor Who Unbound series. Because this collection of "What if...?" stories is completely alternate to the television Doctor's timeline, and exists outside of canon, it's unclear what Doctor he was. However the assumption is that he was depicting a Second Doctor in the story Exile, where he promptly regenerates into a female Third Doctor played by Arabella Weir.
Other actors in the Unbound series portraying alternate Doctors include Geoffrey Bayldon as assumed First Doctor in both Auld Mortality and A Storm Of Angels, the wonderful David Warner as an assumed Third Doctor in Sympathy For The Devil and Masters Of War, and David Collings as an unknown Doctor in Full Fathom Five. At the end of that adventure the Doctor regenerates into the uncredited Big Finish regular Ian Brooker.
Jon Culshaw, famed for impersonating the Fourth Doctor in the television show Dead Ringers, actually played the part for real, alongside Peter Davison as the Fifth Doctor, in the audio story The Kingmaker.
You can find the second part of our look back at the other actors who have played the Doctor HERE.