Being prepared requires a lot of Auditions, Rehearsing, Modifications and Tweaking. If you walk away from an audition and don't feel 100% sure of yourself, you most likely were not prepared enough. I have an audition today! I've been modifing my character for 3 days. I'll even tweak her and flesh her out a little more today. Whatever you're doing, if you're focused exactly on the task, that's preparation. It's a way of being very peaceful and confident in bringing your character to live. If you don't book the role, that's because of something outside your control, a look , a match to the other actors, whatever.
This was a pencil for an educational magazine.