John Simm "had a heart attack" every time he saw his "Doctor Who" disguise.
The 46-year-old actor made a surprise return to the sci-fi saga at the weekend, but before he revealed himself to be the returning Master he featured throughout the episode as a different character covering up his true identity with the help of heavy prosthetics, broken teeth and a scraggly beard, and even John himself found the transformation startling.
Speaking in a behind-the-scenes video, he said: "Every time I walked past the mirror I had a heart attack! I was like "Eugh!" - you forget it's on!"
When John did reveal he was The Master to his future regeneration Missy (Michelle Gomez) he had a goatee beard and he has confirmed that the villain's facial fuzz was a nod to the original Master, who was played by the late Roger Delgado from 1971 until the actor's death in 1973.
He said of the beard: "I thought of it as a bit of an old school nod to old school master."
The actor last appeared in "Doctor Who" in 2009 and though fans were expecting to see him back this series, it was still a shock when he was revealed to be the heavily-accented hospital worker Mr. Razor.
However, the audience shouldn't have been so shocked, as producers previously admitted John had been able to film many of his scenes in Cardiff, Wales,without being recognised.
Speaking ahead of the start of the current series, "Doctor Who" executive producer Brian Minchin said: "We kept it a secret because people in Cardiff are very kind. Simm was there for quite a few weeks, he was actually filming on the
streets and nobody saw him and he was in the studio ...
"We were so thrilled by the idea of it, we were delighted because John loves "Doctor Who" and he loves the whole world of it, so when he heard what Steven Moffatt is planning he was so excited. It was brilliant to have him on set - he's a real joy to have around."
Viewers will see much more of John in Saturday's (01.07.17) season finale as a trailer showed him coming face-to-face with his arch enemy, The Doctor (Peter Capaldi) for a showdown.