The reader or the audience knows something the character does not, so the character's words and actions take on a meaning that he himself does not perceive.
In One, No One, and One Hundred Thousand
Spontaneously, without really meaning to, I then let slip the words: Like me.
The bishop takes "like me" as a manner of speaking, while the reader knows that Moscarda recognizes himself in the madman who wants to fly. The two words mean different things to the one who hears them and to the one who reads them.