@xRoguePegasus hi! as much as i love to give advice (i have examples below), i know that i'm not a professional, and will probably be searching Tumblr for tips on what you're looking for. nevertheless, i hope that my lil tips may be of some use at least :)
fleshing out your characters can be a difficult process in general; besides having to decide whether to fit them in with the norm/stereotypes or not, you always have to place them into a situation. what happens when you place a character with anger issues into a situation where they have to decide whether to be angry at a long-lost friend who is making up for their past mistakes or to be sympathetic and realise that the long-lost friend is at least finally confessing and admitting their mistakes? what happens when a past abused person finds their own cousin on the street getting punched and kicked in a corner? what happens when a homeless person with a dream of becoming an astronaut suddenly receives $1M by a rich person?
not only is personality important, but events, experiences, and likings/dislikings can help emotion and reactions too. the person with anger issues example? imagine that the long-lost friend was known to be a liar and used to never confess about what they did wrong. that might change how the person with anger issues reacts, wouldn't it? they may be more sympathetic & understanding- or they may be cautious and wary instead! let's say that the long-lost friend insulted something very, very important to the original character, like ancient armour pass-me-down or religion. would they still be forgiving? probably not as much, wouldn't you think? try to roleplay the scene in your head, perhaps. maybe even compare characters! imagine yourself reacting to the situation, how surprised, angry, or relaxed you may immediately be. then replace yourself with the character instead - they're as human as you are, after all, right? they make mistakes or assumptions too! but that's all part of the plot, right? :)