Troughton

Patrick Troughton
IS
The Second Doctor
1966-1969

Taking over from William Hartnell in 1966 was popular character-actor Patrick Troughton. The idea that the Doctor's alien physiology would allow him to regenerate all his cells at once, giving a totally different outward appearance, and personality, was a stroke of genius; a completely original idea, that not only enriches the character of the Doctor, but ensured the lengivity of the series.
The impact when the final episode of Hartnell's final story, The Tenth Planet, climaxed with him 'dying', only to be re-born must have been tremendous.
Troughton's portrayal of the Doctor was wisely very different from his pre-decessor. Like the best Doctors who would follow him, the Second Doctor could appear a bumbling fool, child-like on his apparant innocence, but could bring to bear considerable gravitas when the situation demands.
Unfortunately only a third of the Second Doctor's stories still exist in their entirety, due to the BBC's policy towards their archive of TV programmes in the 1970s.

For Doctor Who's first six years, the Doctor's origins had been kept mysterious, but his home planet and people were revealed in Troughton's final story, The War Games. This, again must have had great impact for fans watching at the time.
At the end of this story, the Doctor is put on trial for stealing a TARDIS and leaving his home planet (as yet unnamed) and interfering in the affairs of other planets and races. His punishment is to be forcibly regenerated and exiled to Earth in the 1970s.
Troughton decided to pack in because the stress of working seven days a week all year round on such a popular series was getting to him. He tried to stay out of the limelight, until discovering Doctor Who conventions in the 1980s. He sadly died at one in America in 1987.
Patrick Troughton returned three times after leaving the series, in The Three Doctors, The Five Doctors, and the Two Doctors.