An actor does things every day that no normal person would do. They get up and brush their teeth like normal, but then something strange happens. They start a routine that is unlike anything you have ever seen before. They prep their skin and hair and make themselves available to be anyone. They do stretches and poses to make their bodies limber. Then they do voice exercises to make sure that they sound ready to take on any role.
The more an actor prepares the more confident they can be in their acting ability. They take strides to ensure that everything they do makes the role more believable.
But what happens when an actor doesn’t prepare? They become weak and their confidence is shot once they make a mistake. There’s no bouncing back from that.