I'm not exactly sure how casting works...
I THINK if I'm not mistaking, the process goes like this...
The director, writer, and/or producer or whoever usually writes a character with a particular actor in mind, and I believe that's when the casting director brings in said actors to read for the part to see if the director, writer, and/or producer really do feel the actor is just right for the part.
That much I remember, because I saw a behind-the-scenes featurette for the movie Elf, and the director saying he wanted someone like Bob Newhart to play Buddy's adoptive elf father, and lo and behold, in came Newhart as Papa Elf.
Radio was also a chance to live a dream of having Cuba Gooding Jr. and Ed Harris star in a movie together.