"I need to build my confidence" or "I am shy" are two things I read that many aspiring actors and actresses confess. Of course, being in plays is the best way to build confidence. Speech classes are good as well. Professional actors need confidence in many ways. Of course they need confidence in their acting. If they are not confident that they can compete with the very best other actors, then they are not ready to be actors.
To become and to continue to be successful as an actors, aspirants must have confidence in their ability to introduce themselves to strangers. I know of an actor who worked many years as a professional, but when it came to introducing himself to strangers, he was shy. To advance professionally, an actor cannot be shy in this way. An actor has to network, and an important way of doing this is to introduce himself to people in the business--other actors, directors, producers, agents, and so on. If he lacks the confidence to do this, his chances of success are very limited. The actor I mentioned never progressed beyond playing minor roles.
The aspiring actor or actress has to have enough confidence to strike out into the wilderness of new places where they may know no one. It is often necessary to move to a new city to move your career forward. Having the confidence to do that is needed as well.
Actors have to have confidence in their ability to be interviewed. When they make an achievement, they will be interviewed perhaps locally by the media, perhaps on a national talk show. They must be engaging and charming and interview well.
Finally, the actor or actress needs confidence in their ability to be a self starter and self motivator. Acting is the kind of career that demands the aspirant to keep working on his or her career every day. You can't just sit around and wait for the career to come to you. You have to go out and make it happen. Every day. It is exactly the same as a home based business that requires you to make contacts and try to sell your product every day in order to make money. That is what an actor must do to be successful.
I often think that confidence is part of what we call "talent." Just being able to do the work well is a very small part of being talented. Talent must also makes itself known. And that takes confidence.