Characters and social currency.

Writing characters into your story seems a bit of a no brainer. They are people who do things and you hold the strings as the author.
But characters have more that a walk on part in the book. They need to feel 'real'.
One way to do this is to employ something called social currency.

Social currency is when someone tells you something, gives a facial expression or makes a point and they want something in return. We all do it as we hold conversations every day.
We spin the story to get a reaction. That reaction we are looking for is an emotion.
We tell our story to get something in return. Sympathy, empathy, love, trust, outrage, camaraderie or any other emotion. Look at the face of the listener next time you tell a story and realise you are asking for something in exchange.
So with our characters when they interact on the pages they are wanting some reaction. Think of what they are trying to elicit. How they get that exchange is the challenge for the writer.

Dialogue is a good way to find the emotion in the scene. Characters must be real people with real desires. Most people desire something from their listener. An inflection in the voice, a raised eyebrow, a smirk, a wink, a nod or a flick of the hand all give hints to the listener on how they should react to the news or story.

Characters in books can have the same signals.
Think of a baby who can only go on voice and face signals. They know when you are frowning something is not right. A smile brings a reaction completely different.
We all look for the signals so we don't get the wrong end of the stick.
There is an exchange happening between the teller and the listener. It is social currency.

So when you write your protagonist interacting ask yourself what they want from the exchange. Knowing what they are trying to get from the interplay makes it easier to know their next move, reaction, motivation. Make them act like real people.

If you are still not convinced, then ask yourself this question next time you start to tell someone about something.
Why am I telling this person this story?
? To make them understand your point of view
? To elicit sympathy, empathy etc
? To show them you know something they don't know.
Or something else.

Humans interact for a reason. Find the reason and your characters will seem more rounded, more alive and your writing more real.