Inventing a fiction requires familiarity with the context in which the fiction takes place.
This is true for most designed things: if the design is to come off well, the thing must be situated effectively in its context.
Exposition is always primarily linear. However, even though the tongue and the pen are bound by the limitations of [expressing only one thing at a time], neither the mind of the author nor the reader are bound this way. We understand by compounding facts and impressions. Consequently, linear exposition is always the tracing out of a map, and ideally a non-linear map.
Thus three things become objects for the inventor of fictions.
(1) The form of the thing described.
(2) The form of the description.
(3) The relation between (1) and (2).
Thesis: There are no good fictions or bad fictions per se, but only secundum quid.
The multiplicity of fictions militates against the genus having a single perfection or characteristic ideal. Fictions are edifices of language, which is an instrument of human thoughts, relations, and intentions. They are therefore themselves instruments and artifacts, and should be understood to have as many ends and possible perfections as language allows.
In their accidental features, fictions are perfected by a certain harmony or proportionality which is called "beauty". The beauty of a fiction need not entail any sublimity of subject matter or mystical rumination. What is commonly called "cleverness" in fictions is just a species of beauty—beauty consisting in the elegant construction of a plot device or expression.
Note that parsimony is not the only form of narrative beauty. Expansiveness can be beautiful as well. It is not the length that matters so much as the proportionality. . . a feature difficult to define.
Regarding (2), the way to establish the mood of a place is not by repetition of some abstract claim or detail (which is rather cloddish and usually seems overdone), but to confirm the mood by layering corroborating details on top of each other. "Smiley tapped his foot. Beside him, Guillam let out a small sigh. Connie Sachs was wiping her glasses with a florid kerchief for the third time." etc.
There is a kind of harmony to each of the three components of a fiction. Each one requires its own perfection according to is function. But the mode of excellence required of each component differs depending on the intent of the fiction as a whole.
But what I just called "components" must not be confused for some elementary or primary constituents of fictions. Just three aspects of the process of constructing one. Obviously the pie can be cut in many different ways.
In the communication of a mood, it's important for the author to believe in the mood. To believe that this is clever, or interesting, or sad. The structure of the text creates an experiential world parallel to and partly independent of the world which is being described.
The world of the fiction ought to be known well in advance. The characters of the fiction ought to be known especially well. The same rule applies to the composition of a fiction as to the composition of a factual, descriptive treatise—you need to know what you're talking about before you start talking about it.
Regarding probabilities: events are usually improbable—what happens to people, the concatenation of chance circumstances, these things are usually unusual and irrational, but what people do in the circumstances presented, even if irrational, is usually (almost always) understandable. Human action has its reasons, even if they are not strictly logical. This means that in the construction of characters and backstories, one needs to beware of creating chains of development which depend on or yield highly implausible modes of behavior or reasoning in characters—though, on the other hand there is always a degree of inscrutability to human action.