I remember reading that book. His first book was called "I am not Spock" and was talking about all the stuff that's Leonard Nimoy alone, but this one was about how he'd internalized a voice from his character who would have the Vulcan point of view.
One of the most interesting parts of the book was where Spock pointed out there's no need to fear death because it's non-existence, and you won't be there to care. Considering I read this when I was still in grade school, it was a pretty big revelation.
I wonder how many actors have had to go through a similar process as Nimoy: playing such a culturally iconic character that nobody can see you as anyone else. The natural reaction has to be to resist it at first, but then he came to embrace it and allowed it to become a part of him.
Most sitcom actors run into that. I'm sure Jim Parsons won't ever be seen as anyone else, or for an older example all of the cast of Seinfeld (arguably besides Julia).
I don't even know who Jim Parsons is, so perhaps he'll be a little less affected by that, but Seinfeld is a poor example because the character (and show) is literally named after the actor. Though I'll concede your point stands for George and Kramer.