Both excellent points, It really mixes things up when giving a character flaws such as addictions, PTSD, phobias etc, something that could turn a typically textbook situation into a total shit show.
For example, in DayZ's setting.. You run into a fellow survivor out in the wilderness, after the intial tense standoff, weapons raised, you ascertain you're both non-hostile & nobody is about to get robbed etc, you agree to trade.. After listing what you have each, provisions, clothing items, this other survivor pulls out some medication, which your character is cripllingly addicted to.. Suddenly you dont want to trade, you want these for free, & whatever other meds they might be carrying. Or you can force them at gunpoint to tell you where you may find more. Or in another case, your character may have been slightly unhinged since the disappearance of his brother during the initial outbreak, & this guy your trading with is wearing a green baseball cap just like the one your brother was wearing when he vanished. Can this guy tell you more? Did he kill him? Something you had put behind you is now front & center of your mind & you cant think about anything else..
Maybe you were a moms-basement dwelling neckbeard & simply cannot interact with women, no matter how cordial the conversation, without hyperventilating & panicking. Sounds ridiculous, but in many ways its no different to being scared of snakes, for example. On the subject of animals, maybe you're terrified of dogs, so when wolves attack instead of standing your ground & fighting you abandon your group & run..
I'm rambling a bit.. But giving your characters added depth, both positive & negative will give you much more to consider when interacting with others, & will also ensure that very few interactions play out the same way. Each experience builds your character, & will help you going forward, & feel less like you're just 'coasting' along with no purpose, if you know what I mean.