All sorts of things have killed off the Doctor. In the last half century, Doctor Who‘s unique approach to recasting the lead character has seen him fettled by old age, as punishment, by radiation poisoning, falls big and small, dodgy operations and time itself. There are plenty of in-universe reasons for why the Doctor regenerates, and the outgoing Time Lord Peter Capaldi promises that his upcoming demise will be suitably timey-wimey, but what of the behind-the-scenes reasons that the Doctor has to go?

“While you’re enjoying it, leave,” said Capaldi, when asked why he was leaving Doctor Who on The Graham Norton Show back in April. Some had speculated that Capaldi, a lifelong fan of the series, might stick around in the job for years to come, and indeed, he was reportedly invited to stay on by Chris Chibnall, who will replace Steven Moffat as the executive producer and showrunner next year. But there are all sorts of reasons why actors don’t stick around and play the part forever.

“There is a logic to three years,” Moffat told SFX Magazine recently, on the subject of the average Doctor’s tenure and Capaldi’s impending departure. “And for an actor of Peter’s stature and brilliance, he’s cracked it now. He’s done it. […] And now he can go and do another one.”

In all cases, it’s a demanding role – from the serials of the 1960s to the 12-episode series we enjoy now, production on a series of Doctor Who is invariably a job that takes up nine months out of the year. All but one of the actors who have played the part on television so far have had to make the decision to quit the role at some point, but many of them have had different reasons for handing in their TARDIS keys.