In rehearsal for Cyrano last week the director took a moment to give us a quick, amusing lesson in acting. When the phone rings with bad news, he said, then you should be happy when you answer the phone. When the phone rings with good news, you should be depressed.
By coincidence, I was struggling this weekend with a scene in my play in which a man learns his father has been killed. At first I had the man enter tentatively, unsure of what to say to a woman about the drunken night they just had. Then I remembered the director's lesson and had the man enter bubbling over with happiness, jabbering away about last night -- then the woman tells him the news. Much more interesting that way, especially since the audience knows the father has been killed and they're waiting to see how the man reacts.
By using the contrast of happiness to shock and grief, we can see more clearly what the news means to the character. If the character comes on sad and then learns of his father's death, we can't see it as clearly.
The conflict in the scene helps the actors, also. Instead of just serving as voice of exposition, the woman reacts to the man's happiness. Her job of telling him about his father's death becomes much harder. Every happy utterance burbled by the man becomes more unbearable for the woman. The actress has more to do.
I was writing a scene once, many years ago, that had the worst dialogue I had ever written. I couldn't figure out why. I struggled with the scene, rewriting the dialogue, but nothing worked; it was all flat and dull. After some months passed, when I had forgotten the dialogue problem, I looked at the scene again and realized it needed more conflict. So I rewrote the scene with conflict and suddenly the dialogue sparkled. I was not trying to write better dialogue, just more conflict, but better dialogue resulted. Conflict makes everything better (in drama, not in life).
Why is conflict necessary in drama? Because drama is, as Aristotle said, the imitation of an action. Human action is goal-oriented. Conflict shows how much a character wants the goal he is pursuing. Without conflict the action has no meaning and is not dramatized.
Say a story has a character get in his car, drive to the store and buy milk, the end. You could say that's the imitation of an action, something that happens all the time, but it's not a story. The action is meaningless. People buy milk. So what? But if his girlfriend has told him he better not be late for one more date and as he is buying milk on the way to her house he is hijacked by robbers who want him to be their getaway driver, then you've got the beginning of a story (doubtless a comedy).
If you want to make your drama better, make things worse for your characters.