Ack all you guys put good stuff!!!!!!!!!!!!!!!!!!!!!!!!!! AAAAAAAAAAAAAAAHHHHHHHHHH

Idk if this will be helpful or not....but one thing I like to do with my characters is to just let them show me what they're like. For example, I'll start writing a scene where two characters are talking. And instead of planning out what they're going to say it will just naturally occur to my mind - same with their mannerisms and how they move and everything. And if the conversation gets off track of what I intended it won't matter, since I know that it's my characters who are really making the story, and if I force them to do otherwise they'll seem unrealistic.

So basically, stay consistent, and if you just put them in a situation and write whatever comes to mind then it will help understand them better. 

I don't know if this has been mentioned, but if I have a character with not enough flaws then I like to turn a strength of theirs into a flaw. For example, take confidence. It's a strength, but it can be turned into a weakness, such as arrogance. This works with a lot of good traits, and I think it's a more natural way to give a character flaws. 

Hello, Leafy. Gosh, I haven't been here in a while. I didn't intend on posting anything either, but I saw this and I thought I might give a little advice. Also, all the advice stated in previous comments is awesome! I'll probably use it myself actually. 

This is mostly about flaws, I don't think I have much advice for the other stuff...apologies. Anyway, one trick that I use: Your character's flaws build off of your good qualities. Or their weaknesses come from their strengths. Basically, you take your characters good qualities and then you increase them to such an extreme that they become bad qualities in certain circumstances. Your character is honest? Make them so honest that they frequently hurt other people with their bluntness. I don't know if that was good advice, at all, and you might have heard something like it somewhere else, but I just thought I might drop it in.

Good luck with your writing! 

Sometimes, their strengths can be their weaknesses. Are they kind or trusting? Then they can be naive or too easily used. Selfless? They could bottle things up for the sake of others, or be a little too willing to throw themselves in harm's way without even considering a different plan. Every strength can be turned into a flaw in different situations or uses.