Defining Scene and Exposition
Scenes are the life blood of good fiction. They are the moments of your novel or story that readers will experience, hence the moments they will remember. This means your job is to craft your scenes with care, and it's best to look at them on the sentence level. Essentially, it's a balancing act between scene and exposition.
Scene: Action that occurs in a definite moment in time, set in a definite location. ("Bill walked into the bar.") The verb here will most likely be an action.
Exposition: Explanation. ("Bill liked his booze.") The verb here will be more descriptive.
Think in terms of cinema: if your characters are interacting in real time and if your reader is getting concrete details (that is, details that can be seen, heard, smelled, etc.), then you are writing in scene. If the verb in your sentence is not an action verb, you're in exposition.
The key to an effective scene is the balance and interplay between scene and exposition (some prefer the term "summary" over "exposition"). As a rule, exposition serves to give background and provide context for the scenes. As such, it should be used sparingly, only when absolutely essential.
One way to think of the balance is, again, looking at cinema. Exposition in fiction should work a bit like the way music works in film. It would be awkward to have a blank screen with only the musical score, just like it can be a bit awkward to have huge blocks of exposition in a story. An effective scene would have the action and dialogue as the skeleton, with descriptions of the scenery (or character thoughts, or anything for that matter) fleshing it out.
For example, a balanced scene may look something like this: A couple of action sentences to describe the beginning action of the scene (i.e. someone walks into a bar). For additional color, this can be followed by a few sentences of exposition describing the mindset of the character. Then this is followed by a few lines of dialogue with the bartender. Then a few more descriptions of the building. Then perhaps something more interesting happens. You get the idea. The idea is to mix action and explanation and have them work together.
The point is to begin viewing your work on the individual sentence level. When editing, look at your sentences like an engineer would look at a structure. Where on the page does your scene begin? Where does it end? Which sentences convey action, and which ones are expository? Which sentences are advancing the story or character arc, and which are just fillers?
An easy tip at this stage in your writing is to color code your sentences on your word processor: sentences that are concrete scenes in one color, exposition or summary in another. This will give you a quick visual of the interplay between scene and summary. If you have a huge block of one color, you may need to revise.
In a general sense, the amount of exposition you can get away with is proportional to the length of the overall work. A seven-page short story may have no exposition at all, while a five-hundred page novel may have twenty pages of exposition just in the first chapter. The key is to keep exposition quick, colorful, and relevant to the scenes of your story.