One of the most consistent comments I get from my beta readers is how real things always seem, especially with my dialogue. And I would love to give you guys a step by step of how I do that but the truth is, I just do. When I write, I see the scene both through the eyes and mind of my protagonist, but also as if I am just standing in the room watching. Those two images overlap for me, but it's the latter where so much of the realism comes out. Because that's where the little things show up.
The little things like taking a step forward in the middle of an intense conversation, or seeing the person the protag is talking to look away as their arms cross. In a fight, does your boy try to touch his girlfriend, or does the dad reach out for his son, think better of it and let his hand fall by his side? What are the small mannerisms of each person in the room that you can convey naturally, without repeating the same things over and over? The things beyond the sighs, and the eye rolling, and the shrugs?
That, I think, is where much of the realism in my writing comes through. I don't waste time on elegant descriptions. I admit, I don't describe how the blue of the wall matched the deepest oceans, or how the way the light glinting off the blond streaks in a girl's hair were like angels singing. Sure, I mention that the sun was hitting the girl's face, or the blue walls contrasted to the anger in a character, but I focus on the people more than the surroundings. My style is to give just enough for the reader to see it and let their mind go, because unless those blue walls truly have a huge importance in the book, I don't waste my reader's time explaining about them. When it comes to word economy, at least for my natural style, I bypass the in depth, flowery descriptions and go right for the kill.
Take the picture here as an example. When you read about this kiss, however it plays out, do you want to know what they were wearing, how the umbrella is black and not blue, and the fact that the flag in the background is red? Or do you want to know how tightly they held each other, how they tasted, how long they paused before launching into each other's arms? Me? I'd rather know the latter, and that's what I put into my books.
So there you have it. My little guide to realism in dialogue and description. Now take all that, couple it with just the normal dialogue and talking between your characters, adding in all those small things, and you've easily built the entire scene in the mind of the reader, without doing a paint by number for them.