One of topics discussed in writers conferences and the How-To books from the staff of these events is the “show the reader, not tell the reader your story” spiel. The why is left to the virgin writer to figure out. It’s like that “obvious to the casual observer” bit that your physics prof at college used. Well, you need to know what causes the problem and how to approach the fix.
Narration dulls the spirit of any fiction. It can't be avoided entirely unless the story is only a rocket blast of dialogue, but it should be tamed so it doesn't drag the plot. Dialogue is the magic bullet in all action, thriller, or personal feeling stories. Dialogue shows the reader by opening up life and letting you participate in the protagonists struggles.
Can you imagine how quickly a story about Einstein would melt into boring documentary as a narration, although, there have been several very heart felt stories written about him in first person and third person dialogue showing you his life. Sorry, I seem to hitting on physicists in this piece.
Editors will also tell you that a story has a momentum, a pulse, a feeling of undulation that makes it more powerful. If not there, or erratic, it throws the reader off or may cause them to put down the book and do other things. To me, a lot of this is editor paranoia, the fear of low sales, but the point should be looked at as a writer and changed if you feel it too.
So, how do you control the fast pace of dialogue and the slow stretches of narration? Think about it five minutes and you can answer your own question.
By paying attention you realize the two oppose each other, therefore, balance them for effect. When you need to slow the story, use narration or internal monologue. When the story's excitement is what you are striving to achieve, use dialogue with minimum interference of attributions. Arrange scenes to minimize characters present so the dialogue can fly.
Check out any Michael Crichton novel (Jurassic Park, Prey, etc.). When the characters are being chased by beasts or bugs, the dialogue screams, rarely stops for breath, you’re on the edge of your seat and eyes bulging. But you can’t sustain that or you’d give the reader a heart attack. (I’ve actually had to stop reading and get a glass of water. His stories scar me to death.) Notice - go on, read one and see what I’m talking about - he skillfully places a narrative chunk that slows the pace by explaining things, sometimes with passive dialogue and some just narration. But it sooths the reader, and at the same time it sets up the reader for the next slap in the face action.
This rollercoaster ride is why people are thrilled with his stories. Like great composers that work music into a constant undulation of sound and rhythm, writers must learn to do the same.