One of my regrets in life is not being able to do what I really wanted to do: teach acting in a college setting. I wanted others to learn what I had learned on how to be a good actor. I have to say though, in all humility, I am a great actor. Anybody can be a good actor, but a great actor is developed from the inside. Some say that one is born to be a great actor. You can learn the basics to be a good actor. You can learn how to sit and how to stand. You can learn an accent or even bits of a foreign language. You can memorize lines and repeat them in a scene. One thing a good actor can't do is separate himself from a character.
A character has to be developed to be believable. You have to know what the character likes to eat; who were his parents; what was his childhood like; who were his friends growing up; what was his education like; did he finish high school; did he go to college; where does he live; does he live in a house or an apartment; does he work; what kind of job does he have; does he like his job; does he like the people around him; does he have any bad habits; what kind of car does he drive; is he married; has he been divorced; does he have any children. The list is endless in developing a character. Some of these questions can be answered from the script. Others might have to be answered in your own imagination.
I can count on one hand the number of great actors I have seen in my lifetime, either in person or on the screen. They are a rare breed. The key to telling the good actor from the great actor is if you forget that you are watching the actor, whose name you might know, and you see the character he is portraying. If you just see the actor, he is just being good. If you forget about the actor's name and dwell on the character, he is being great. Not all great actors are always great. Most will tell you that sometime in their careers that they have just done a mediocre performance.
I am super-critical in my work. I have to be to get better. What an audience member might see is not what the actor sees about himself. That's kind of why I don't like seeing myself on a TV show or in a movie. I am always seeing something I could have improved upon to make it better. The director might have thought it was all right, but I didn't necessarily see it to be all right. I take the acting experience very seriously. That's why I am a great actor.