Why Every Lead Actor Left Doctor Who

13. William Hartnell

When William Hartnell agreed to play the Doctor in the early '60s, the concept of regeneration didn't even exist. This was a family program about a mad man with a box – nobody involved thought that Doctor Who would spawn an unprecedented 60-year run.

But sadly, several problems necessitated a plot device that would allow the show to switch leads.

Hartnell was no spring chicken, and as his health deteriorated it became difficult for him to keep up with Doctor Who's gruelling production schedule. He suffered from arteriosclerosis, a condition which hardens the artery walls and restricts blood flow, which would often cause him to forget or mix up his lines.

Combined with the fact that Hartnell didn't get along with producer John Wiles (who replaced original producer Verity Lambert in 1965), the idea was floated to recast him during The Celestial Toymaker. The Doctor would be invisible for most of the narrative, only to return with a brand-new face.

Though this idea wasn't used here, a similar concept of "renewal" (the term "regeneration" wasn't invented until later) was devised for The Tenth Planet, by which point Hartnell's health had declined to such an extent that it was difficult for him to keep going.

The First Doctor changed his face on 29 October 1966, and Doctor Who continued under a new lead actor.