He was not the original voice. As he told the audience at the 1984 Denver Star Trek convention, early in the '70s he learned that the position of voice actor had come open, and he auditioned for and got it.
I met him in the hallway on Sunday morning of the con. He was friendly, gracious and all around pleasant. And the direct opposite of "the camera adds ten pounds." He was HUGE. Not fat, any more than he ever was, including playing Harry Mudd. Just HUGE.