A standard rule you may hear in writing is "Show don't tell". Actually, you will hear this applied for stage or film writing. Essentially, it's easier for your audience to "see" what they need to know about a character or situation than it is to have a character describe something. Overstating the obvious can insult the intelligence of your audience.
In my previous post on exposition, I used the concept of "telling" about a character named Uncle Henry. The telling came through dialogue between two characters in the guise of a casual conversation. It was established that Henry drank, gambled and was injured during the war. Henry's character background was established before he ever entered the scene. Now, if I were writing a play or a film , I could save some of that 'exposition' by simply allowing his character to "show" some of those details. How? Well, Uncle Henry could enter the stage with a bottle of whiskey in his hand and he could slur his words as he spoke. He could walk with a limp or even use a cane to move about.(Henry drinks) He could be wearing an old army jacket or coat with the Purple Heart medal attached to it.(Henry was in the war) Henry could ask the other characters to loan him money because he was trouble with his 'bookie'.(Henry gambles) Any of these elements would "show" the audience detail of his character without needing to establish these facts in expositional dialogue.
As a writer, one question you may find yourself asking yourself is: what do I really need my audience to know? Do I have to "Show" or "Tell" them everything? What it important to the story and what is just fluff?
That is a question I wrestle with all the time. One great example of a major 'detail' being left out is in the Cohen Brothers film Barton Fink.
Halfway through the film, a female character is murdered and the head is missing from her body. Later on, Barton receives a mysterious package that appears to be a box. The audience is not told or shown that the victim's head is indeed, inside this box, it's only an unspoken idea the viewer 'assumes'.
This assumption carries all the way to the very end of the film, where Barton is sitting on a beach and nearby character asks him, "What's in the box?" and Barton replies, "I don't know." End of film. We never truly know.
A writer may choose to omit certain facts or details. Sometimes the audience's imagination is far more powerful than what it can be shown or told.
I personally think what made "The Blair Witch" frightening is what we didn't see. The great Japanese film director Yasujiro Ozu was famous for not "showing" or "telling" the audience certain details. For example, a story will lead you up to a Wedding and the very next scene will be years later, without the wedding ever being shown. It was implied, the viewer knew about it, why then waste time showing you what you all ready knew?
When it comes to exposition and telling your audience what they need to know, deciding what they need to know or in some cases not know can be a sticky wicket.
The best way to determine this, is to ask yourself: does this detail add to the story? Does it help move everything forward? Or is it just you, the writer, showing off your descriptive skills. Does James Joyce really need three pages to describe a character walking around a corner? (Well, that's what made him James Joyce isn't it? He mastered the art of "telling" which is can be tiresome in a visual medium.)
When it comes to exposition, do we really need to know "What's in the box?"
I don't know.