Avoiding "as you know" Explanations

In The Writer magazine, December 1989 issue, Jeffrey Sweet wrote an article on "An Object Lesson for Playwrights." In it, he writes:

You can often dramatize what is going on between your characters through the way they negotiate over an object.

He gives an example of a man stopping a boy as he goes out the door. The man demands to know what the boy has in his hand, finds out it's a key, wants to know where the boy is going, and then demands the boy give him the key or he'll ground him for another week. From the example, it's obvious the man is the boy's strict father, the key is to the family car, and the boy is currently grounded and is resentful about it. All of this was conveyed by how the two characters interacted around the key instead of by the author explaining the situation to the reader.