Speaking to the Daily Telegraph, Capaldi opened up about the sheer level of energy required to play The Doctor on a regular basis. He was blunt in stating that even going into his second series, he still feels more is to be learned:
"I don’t feel I’ve nailed it yet – from an acting point of view, I mean. I don’t yet feel that I know how to do this. Quite who the Doctor is remains mysterious to me – which is of course as it should be – but one of the biggest challenges that I’ve found, and am finding, is that you have to sort of be able to spin on a penny.The magic of playing The Doctor is still not lost on Capaldi, a life long Who fan. He continues:
“You have to be able to go from pantomime to tragedy, from domestic to epic, within a single scene.''
“What I wanted to do, I suppose, was remind people of the alien-ness of the Doctor. Doctor Who isn’t a human being, you see. He’s a creature of the cosmos. His social skills aren’t great. He doesn’t care very much if people like or dislike him, because people aren’t his thing, you know?”
“It might just be my Scottish melancholia, but the very first day I found out I’d got the job, I started to feel sad that one day I would not have it; that there would come a day, in the not too distant future, that I wouldn’t be Doctor Who any more. And that is why I try really hard to get as much out of it as possible.The inevitable departure from the role is something Capaldi clearly fears and will remain a sore spot until that day ultimately arrives. Yet apparently that is not the only sore spot; the knees, a problem shared also by his predecessor Matt Smith:
"Because one day I’ll just be an overweight has-been, trying to get a meeting with Jenna Coleman and being ejected from a Doctor Who Convention in Bolton for being drunk and disorderly. I mean, this is surely my high point, isn’t it?’”
"It’s something to do with running down corridors and turning round very quickly to deliver lines”.Source: Radio Times