SPOILERS: If you found those skulls eerily familiar, there might be a reason for that…

Morbid as it may seem, Doctor Who fans have just had a glimpse of what Peter Capaldi’s skull might look like…

Viewers watching this week’s episode, Heaven Sent, discovered that each of the hundreds of skulls seen lying on the seabed at the foot of the castle represented a time the Doctor had tried and failed to complete the tasks set for him by the mysterious force that had transported him there.

What they may not have realised is that Capaldi himself modelled for the skulls, which were created by working backwards from a cast of his head, a bit like the way forensics experts rebuild a face in clay, but in reverse.

“We had Peter in for a lifecast for episodes three and four [Under the Lake and Before the Flood] as we’d made prosthetics for the Ghost Doctor,” Kate Walshe, SFX producer at Millennium FX, told RadioTimes.com. “As we already had his lifecast, [producer] Pete Bennett and I discussed the possibility of making a skull based on Peter’s own face.

“We cast a version of Peter’s face out in clay and, following the contours of his distinctive bone structure, pared the form back to reveal what we imagine may be Peter’s own skull.”

Once you know the truth, it’s easy to see Capaldi’s high cheekbones and narrow face when you look at the skull, but Walshe said it had been important not to make it too obvious from the outset that this was the Doctor as it would have spoiled the big reveal that he was repeatedly dying and being brought back to life.

“Once you strip away the flesh of someone’s face they lose enough of their appearance so as not to make it immediately obvious whose the skull is,” said Walshe. “This was really important for [director] Rachel Talalay and Pete Bennett so as not to give the game away too early.”

Judging by fans’ shocked reactions during Heaven Sent, they succeeded in keeping the mystery of the Capaldi skulls a secret…

Doctor Who series nine concludes on Saturday 5th December at 8pm on BBC1