You should be able to learn a character's heart through what they say.
Try to make conversations believable and not too forced, more fluent.
Make your dialogue well formatted and mature, if it is childish, it will ruin the story's content.
Include a character's conversation as well as their unspoken thoughts so the readers get a better understanding of the character that other figures in the story do not.
Try to make each character sound different from the rest.