Or Peter Falk as Columbo. James Garner as Rockford. Without those particular personalities, those shows would not have worked. One thing that hasn't changed about Hollywood is that every audition attracts hundreds of actors hungry to break in. Actors we hire almost always have been in other shows, films or on Broadway.
Actors build up experience in smaller roles and eventually move up to larger ones.
The auditions themselves reveal fairly clearly who would and who would not be good for a part. Some actors come in so eager to please that they play to the casting people, and it comes off as insincere. I've always thought the best acting was the kind that seemed effortless. When a person is right for the role, they seem to be playing themselves rather than a character, and you forget for a moment that they're reading someone else's words. It's pretty obvious when that happens, so often it isn't difficult to choose who is best for the part.