I've missed more than 9000 shots in my career. I've lost almost 300 games. 26 times, I've been trusted to take the game winning shot and missed. I've failed over and over and over again in my life. And that is why I succeed.
Before you start production, you have characters you have created without actors in mind, then all of a sudden you've got actors. They bring an enormous amount in creating these characters, and creating the dynamics between the characters that you've written.