One thing I forgot to mention. This is something some will likely disagree with, but... Use improper grammar. Humans are imperfect. In dialogue, a character shouldn't use flawless grammar and wording and such. It should sound natural. For you, it's thought out and planned and edited. For them, it's quick, fast paced, in the moment. The characters aren't going to have time to stop and edit there speech while in the actual conversation.
This is something a lot of fanfic, indie webnovel, etc all fail on. they either over edit everything, including the dialogue so it sounds rehearsed by bad actors, OR they fail to keep it legible.