You're discounting the ability of the actor to bring something to the role that the director couldn't envision themselves. The whole point of having casting auditions is so the director can find an actor that either fits their own idea of the part or blows them away with something transformative that they could not have envisioned before seeing it. Not every actor is capable of that and not every part calls for it, but when it does happen it creates something best described as the magic of cinema. If this weren't the case, the phrase "Hello, Clarice" would be completely meaningless to everyone who reads this post, and yet there are some for whom it is not merely memorable, but evokes an entire range of emotions and even causes them to read it in a different voice. Tell me that Anthony Hopkins was just a middle man with a straight face.