character writing and dialogue kind of go hand in hand, and can take some time to develop your voice with. you'll have to go and speak to people, or listen to people speak at places like coffee shops and bars and the like, and pay attention to how people speak. like a close reading. i worked at a radio station for a while, and that's where it clicked for me how to write for the diction of another person. if you communicate through chat and text a lot, i'd also recommend going back over any saved conversations or logs you might have. that informal style is closer to the rhythm of the writer's speaking voice.
as for the advice to go out and listen to other people speak at those public places, it is basically eavesdropping, and i certainly wouldn't feel comfortable writing down a stranger's conversation. you can do just as much by paying attention to your friends, family, and other people you may speak with in your daily life. where people may pause to think, or interject with a thought on the side, or talk over each other. the grammar of conversation is quite a bit looser and less formal than prose. which, whatever you decide to write, even the most fantastical or abstract thing relies, to an extent, on believable characters.
as far as making it believable goes, you may find yourself actually heightening or exaggerating real conversations to avoid presenting something as a caricature, or collapsing the traits of several people you may know to build a single character personality. it becomes important to think about the traits of the character in relation the themes, conflict, and/or fundamental underlying argument(s) of the story idea. i don't quite remember where i heard it, but i feel a good descriptor for dialogue is "two arguments in opposition". this can count for the whole of a story, or even just for the moment of a given scene, like how we refer to positioning and dominance when talking about a scene from a movie or play - both the physical positioning, character motivations, and things like where that scene leads to and from all inform and relate to the theme(s) of the story. in paying close attention to these kinds of elements we avoid "telling" instead of "showing" (such as: the dreaded exposition dump), and both better flesh out the ideas of the story, and provide the whole of the project with cohesion.
