Far too many novices write dialogue that’s too on the nose. That means the dialogue either explains too much of the backstory or that the dialogue directly states what each character wants. Both are unrealistic.
If you want to write more realistic dialogue, start by writing no dialogue at all. Write your scene using nothing but physical action.
For example, suppose your scene involves a couple on the verge of breaking up. Poor dialogue might have both people arguing and accusing each other of flaws or problems. While yelling and fighting can be visually interesting, it can also be dramatically boring.
Instead, try describing your scene without any dialogue at all. Then decide if two people were going to break up, how would they move in the scene? Obviously they wouldn’t hug each other or hold hands. Instead, they might simply ignore each other while constantly getting in each other’s way. Or one person could be preparing a meal to earth alone while another person decides to listen to music really loud to annoy the other.
By physically showing how two people might behave on the verge of breaking up, we get to slowly unravel a mystery and piece together clues that the two people no longer love each other any more. By actively engaging us in figuring out the true situation, silence and physical action invites us to eventually conclude that the two people are on the verge of breaking up.
Notice how much more interesting this can be than just hearing two people telling each other they want to break up?
Besides silence, another technique is to talk about anything but what’s on each character’s mind. The couple might initially seem to be talking to each other, but one person could be talking about how the plants in the windowsill are dying because they didn’t get enough water (symbolizing the relationship) while the other person could be talking about how the apartment is too old and cramped that it makes living there unbearable (also hinting about the relationship).
Notice by talking about the situation without directly talking about it also makes an audience engage in deciphering the clues on what’s really happening, which is far more interesting than two people just talking about breaking up.
Silence and indirection dialogue are the two best techniques for writing realistic dialogue. In real life, people rarely say exactly what they mean and stories told through screenplays shouldn’t do so either.
Keep your dialogue real by talking about anything but what’s really happening, and you’ll find your dialogue will feel more interesting and compelling as a result.