Chamberlain -- WRONG!!!!
The mini series would have been so much better with a different Blackthorne.
At the time, though, Chamberlain was sort of the King of the Period Mini Series so he always seemed to get these roles despite the fact that, although good-looking enough he was boring and wooden as an actor. He was OK as the priest in Thornbirds -- but not for Shogun.
He was just WRONG as Blackthorne. Blackthorne was basically an Elizabethan buccaneer of the Francis Drake type. Quite a bit more learned than the average buccaneer, of course, because of his training, and could be both silky-smooth and rough-hewn, depending on what was needed, but he needed to have the underlying toughness.
Chamberlain had the smoothness, but little of the sheer toughness and virility needed for the role. Do you see Chamberlain controlling a bunch of restive sailors across thousands of miles of ocean? Or imposing his authority by sheer personality and ingenuity on a violent and militaristic culture, once he reaches Japan?
I remember reading somewhere that Sean Connery was considered for the role. I don't recall whether they decided not to ask him, or whether he turned it down. It would have been so much better with Connery.