One of the greatest challenges I find when I am writing, is the part where I play God. Meaning - when I create a life. Make a character. Flesh them out and let them loose.
The first thing to remember is that you have to be careful when you mold your characters. Every event you throw at them will twist and shape them throughout the story - too hard and they will become cold. Too soft, and they will become snivelers. If you make their past too traumatic, they will become catatonic and socially unacceptable. If you make their past too happy and too perfect, they will become naive children who will be so easily harmed.
This applies to many things - Role Playing games as well as stories. Anything where it requires you to fashion someone from thin air.
I have most certainly thrown characters together on a whim - on occasion they become amazing. In fact, a select few of my favorites were done just like that. I was given 5 minutes, a character sheet, and they said 'go.'
The main thing I have trouble with is refusing my own influence to bleed into the person I have just created. Granted, there are just certain situations where I have NO IDEA how I would, personally, react. But in normal, every-day events it is the hardest. what if the character is a smoker and you are not? You would refuse a cigarette if offered, however, your character would most likely accept. Or let's get even more basic. Your character is not as intelligent as you are.
That's a doosie.
YOU know exactly what happens if you piss on an electric fence. Your character however...?
You know I think I started this post out very differently in my head. So I'll end it with something random.
Writing prompt Idea:
Look through your character's desk drawer. Create knick-nacks and throw-away items they possess. Craft a short piece based on them cleaning the desk drawer out and what they find. You can tell a lot about a person by how neat or messy their drawers are, not to mention what is inside them.