So, if an actor hears about a part that's being cast and is perfect for it, that doesn't mean they'll ever even get a chance to audition. Most of the time they won't. First of all, much of the time you have to be submitted to casting through an agent, or if not, then know about and get invited to audition another way, by a manager or directly through the casting director.
For me almost every role I have ever booked has been directly through a casting director. They have always seemed to have much more imagination for what I can do than an agent who seem to want to put me in a certain category and "sell me" that way only, which is common with many agents and most actors don't take it personally.
Every famous actor at some time or another had to audition for that role that gave them their big break and many successful actors who are celebrities in their own right still have to audition for certain roles. Most of the time for them it's in the version of a screen test....with the director present and their reading taped rather than just in the casting office. Also, most roles, even those that are just a few lines usually require at least two auditions before booking. There's the introduction audition and then the "call back" as it's referred to. Lead roles are usually multiple auditions.
Below are seven auditions of famous actors for roles of which they all landed the part. (At the bottom of this article are the text links to YouTube for media players that do not display embedded video.)
Robert DeNiro...."The Godfather" Audition
Rachel McAdams....."The Notebook" Audition
Seth Rogan and Katherine Heigl screen test for "Knocked Up"
Carrie Fisher..... "Star Wars" Audition
Beyonce...."Dreamgirls" Screen Test
Aaron Paul.... "Breaking Bad"
Links to YouTube for the above auditions:
Robert DeNiro....The Godfather Audition
Beyonce....Dreamgirls Screen Test
Aaron Paul.....Breaking Bad Audition
Henry Thomas.....E.T. Audition