If James Marsters had been recast as a different character (and a good guy!) in Angel it would have been equally weird. Or to pick a different fandom, if Joshua Cox or Andrea Thompson had been recast in Crusade, it would've been strange. It'd add a layer of cognitive dissonance that would pull me out of the story. Instead of thinking about what's going on, I'd be reminding myself that the actor is playing someone else now. As I had to repeatedly when Benedict Cumberbatch was fighting Zachary Quinto in the latest Trek movie. (That's also not meant to knock Cumberbatch or Quinto particularly, though Cumberbatch seemed almost typecast, which makes the distinction harder.)

It's easier to set up the divide between actor and role when it's in totally separate universes. Patrick Stewart will always be Captain Picard . . . but also Professor Xavier. However, I can't imagine anyone ever taking seriously a Trek movie (or new show) recasting Stewart in some other role in the Trek universe. It would just be wrong.

I admit I have not seen Capaldi in anything else; it is possible if I had I would not have this concern in his particular case, and certainly Capaldi's role in Children of Earth was not Picard-level important. But it was important enough that the face could immediately be called to name, and role was memorable. So it's enough to give me pause at least -- I'd much rather have had an actor who I'd never heard of so that there'd be this totally new Doctor to get used to.

I'm just worried that seeing him as Frobisher is going to take away the emotional punch of the first few episodes. And if that happened, it would be a shame.

(Though no matter what, obviously, I'll just have to get used to him one way or another, and in a few years it'll be someone else. Probably Cumberbatch the way he's ending up in everything.)