What do you think defines you? Your looks? Your personality? The Starbucks coffee you drink in the morning?
When writing characters in a story, you get to pick whatever traits you want, and make the person as suave as possible or as frail and insecure as you want. The great thing about writing is you are in control. The ideas may come out chaotic, and it may take several rough drafts to get to that final product, but in the end, you are the one who shapes everyone and what happens.
In real life, this is not so simple. Sometimes you only know what defines you, and not necessarily the people around you. People are really good at hiding flaws and traits they do not want you to see, and thus you can only interact with them based on what you know and observe.
It’s hard sometimes to discover what really motivates people, what their agenda is if they even have one. The one thing I try to stick with when I interact with people is this: smile and be kind. You don’t know what kind of tragedy that person has been through, or what they might still be going through.
In a story it is easy to know how your characters will react because you know everything, but real people sometimes hide their pain and you don’t know how to react. Your co-worker that’s angry all the time? Maybe they have some issues at home they are dealing with and is taking it out on people at work. That person that seems to keep to themselves all the time? Maybe they are dealing with being insecure and not in control of their life. These are just generalizations, but you get the point.
Again, smile and be kind: you don’t know what battles people are fighting.