This question asked about actors who appeared in Doctor Who as various characters and then went on to play the Doctor. This question is about the First Doctor story The Chase. In this story, Peter Purves played two characters – Morton Dill (in the episode where they land on the Empire State Building) and Steven Taylor (the final episode on Mechanus, where he becomes a companion). Is there are in-universe reason for this? Was Morton Dill supposed to be an ancestor of Steven Taylor? Out of universe, was this cheapness, or perhaps someone didn't turn up?
Edit: Coincidentally I am just reading "Who's There? The Life and Career of William Hartnell" by Jessica Carney. On page 167 she writes "One day Bill [William Hartnell] and Maureen O'Brien [Vicki] hatched a plan. They decided they liked Peter Purves when he did a one-off character role in an early episode of a Dalek story, 'The Chase'; so they went to the producer and suggested him for the role of Stephen Taylor ....."