<LOL!> That reminds me of a confrontation between Noah and Nathan that took place, IIRC, in the drawing room at Collinwood. Noah was giving Nathan an ultimatum and he made a threat along the lines of, "If you don't, I'll make sure that everyone in this house knows why I'm here!" However, when watching that episode, I honestly thought Noah was going to say, "If you don't, I'll scream!"
Apparently, Dan Curtis's wife saw Slocum in a play and raved about him, thus leading to his being cast on DS. I agree that he was miscast as a tough guy, but one of the things I love about DS is that even the bad casting choices somehow seem to work.
Regarding Harry Johnson, I seem to recall that the actor who replaced Slocum (Edward Marshall?) had a speaking voice similar to that of Fred Schneider of The B-52s. Again, not a typical tough guy...