Casting a voice artist for a show involves knowing what type of skills from him or her before you begin. Hiring a voice actor and just hoping everything will be OK is foolish, although sometimes good results to occur. But do you really want to bet your project to be governed by a happy accident? Even a picture-perfect script is not going to sound like a good script if you cast the wrong people. If the voice over talent doesn’t deliver them right, then the project could be utterly ruined.
Is the character a weird creation? A pirate with a moral code? Someone like that is going to need a voice actor that can capture the apparent contradictions in his character. But the voice talent is not going to understand those complexities unless you reveal it to them first.
Of course, if the voice actor is an exceptionally talented and creative sort, you can just give him some lines and tell him to create the character, but then you must be willing to live with whatever happens. Some directors enjoy working like that, but you have to do that sort of thing deliberately, not because you don’t know your head from a hole in the ground.