Character Clues
Character Analysis
Speech and Dialogue
Because King John is a play, the main way we learn about its characters is from how they speak. For instance, King John's belligerent speeches to King Philip at the beginning of the play—and his weak whimpering at the end of the play, when he delegates authority to the Bastard—show us that King John is a shifty character, subject to serious mood-swings.
From the Bastard's obscene puns and gratuitous insults, we can tell that he is a boisterous outsider, who doesn't quite fit in with the high-and-mighty social circles where he currently finds himself. From his tangled displays of logic-chopping, we can tell that Pandolf uses learning (such as it is) as a weapon, and that he has a love of manipulating people. From her frantic, disordered speech in Act III, we can tell how much Constance loves her son, and how deeply grieved she is to have lost him.
The ways in which language reveals character in this play are practically endless; these are just a few hints to get you going.
Actions
Just as important as speech and dialogue are actions, which often speak louder than words. Even though both the Bastard and Austria talk big and like to throw insults at each other, when it actually comes to fighting, the Bastard proves by his actions—killing Austria—that he is, in fact, the better fighter. Even though Hubert promises King John that he will kill Arthur, and then tells Arthur to his face he will blind and kill him, the fact that he can't go through with the act shows that he isn't as hard-hearted as he was trying to make people think he was.
Even though King Philip talks a big game about how he wants to become loyal friends with John and patch up their differences, when push comes to shove, he breaks the alliance. Thus, you could say that speech and dialogue sometimes just give a hint about a character's nature. To judge if that hint is true or not, you have to look to the character's actions.
Thoughts and Opinions
The mode of characterization through thoughts and opinions is similar to characterization through speech and dialogue, but different from it in an important way. The similarly, of course, is that both have to do with speech, but the difference is that while speech and dialogue have to do with how characters speak, thoughts and opinions have to do with what they say. For example, when the Bastard makes long speeches about how he wants to rise up in the world and about how Commodity is what makes the world go round, we can tell that he is a cynic who is free from rosy illusions.
When young Prince Henry expresses amazement that the dead King John can have changed in an instant from being a king to a senseless clod of earth, we can tell that he has a sensitive and philosophical nature. It's true that this mode of characterization doesn't come up all that often in the play—the characters are usually too wrapped up in the action to have time for thinking and forming opinions—but on the rare times it does show up, it's that much more important a window into character.