Building atmosphere

When you direct a play, you have to think about what's usually called the mood line.  You can check this against any good movie and see it in action.  Works like this:

Opening: pre- and during credits, your director tells you what kind of movie this is going to be, using sound, light and images.  He plays on your emotions by setting the atmosphere.  Example:  the drone of aircraft and a gradual increase of light to an image of a squadron of bombers on the runway waiting to take off is the opening to Catch 22.  Sometimes, you'll get silence and then building images, or birdsong, or a cityscape accompanied by music appropriate to the genre.

With a novel, you have the added advantage of being able to introduce the character's thoughts.  But you can also use visuals, sound and exterior forces (weather, for example) to create your opening atmosphere.  Think of the emotional impact you want to have on your reader, and then build the word picture to give them that.

The mood line goes on from there, running alongside your plot-line to build in highs and lows, changes of pace and tension, shifting into high gear for the climax and coming down for the denouement.  Atmosphere at any point contributes to the mood line.  I recall an editor critiquing the "relentless pace" of my then novel, giving the reader no time to breathe.  She was right.

The key is to surprise your reader, but make the progression from one mood to another logical.  Be careful of shoving in a completely inappropriate scene that will confuse genre on your reader.  For example, you can't expect to suspend disbelief if you have your rom com heroine kidnapped at knife point, unless you're writing a comedy thriller.

Because in the last analysis, genre to a large extent dictates the atmosphere.  A great deal depends on fulfilling reader expectations.  On the other hand, any drama has its lighter moments, and vice versa.  Try not to stray too far at any point from the basic atmosphere you created in your opening, because that tells your reader what sort of story to expect.