Holden Caulfield in the Catcher in the Rye is talking about his dislike for actors and the theater describing all of them as phonies . This comes up because he bought tickets to see The Lunts with Sally. His dislike for them is because the actors don’t act like themselves, they pretend to be someone they are not. In the book he explains, "I hate actors. They never act like people. They just think they do". He is justifying his classification of actors as phony by explaining how they themselves even get mixed up in all their lies, to the extent of believing themselves.
The specific actor he is talking about is Sir Laurence Olivier who played Prince Hamlet in the 1948 rendition of Hamlet. He said “if any actor’s really good, you can always tell he knows he’s good, and that spoils it.” which is Holden explaining that a cocky shows on stage and ruins the acting, because the person the actor is playing isn’t necessarily very full of himself. He also explained how he played Hamlet wrong, Hamlet is, in his eyes, supposed to be a trouble young man when Olivier plays him like a general.
Holden may have been more upset that Olivier didn’t play Hamlet right because he relates to Hamlet a lot. They are both troubled with deaths of family members, which leads to both of them being sad and depressed characters. They also are faced with big life questions that they themselves can’t quite answer, which they have to deal with, never knowing the answers to their biggest wonderings.