Begin in the middle. It is one of the first pieces of craft advice that I remember ever receiving. The best way to generate interest in your reader is to start in the middle of action. Drag the reader along with your characters into a situation that is compelling emotionally and intellectually.
Beginning a scene with dialogue is a form of this. The conversation is the action. So, how do you do it? Start in the middle of the conversation and let the reader catch up! Make them question what was said before and make them care about what is going to happen next, what will be said next.
The best way to create suspense, increase tension, and build momentum within a conversation between two characters is to shorten your sentences, keep your descriptions brief, and make the responses to questions abrupt but informative.
Is your character talkative? Is he or she a ranter? Are they unreliable? A fast talker? Do they exaggerate? Do they communicate well? Are they impatient? Are they educated? Are they slightly more crazy than anyone wants to admit?
All of these questions can be addressed if you are careful within dialogue -- which is very important when it comes to characters who will never have a point of view in your story. Secondary characters, especially, can be filled out with the words you chose for them to speak.
1. Make your characters distinctive by varying the way each speaks in your novel.
2. Don't use the same phrases and pacing for all of your characters.
3. Don't ramble in your narrative or in dialogue unless it is for a PURPOSE.
Write Great Fiction-Dialogue by Gloria Kempton
Characters, Emotion and Viewpoint by Nancy Kress