Adventuring is dangerous. Really dangerous. It's like being in the special forces, and sneaking across enemy lines in an active war, so that you can take out squads of enemy troops in their home territory. For ten years.
Friends are few. Comrades are fewer. Those who become adventurers are either crazy, desperate, or both.
Adventurers are an eccentric bunch. What makes a good character interesting are the small details, and hidden motives. To flesh out a character, here are some questions you should consider as you build yours. These are not mandatory. Many of these should only come up rarely in game. Some of them never. The purpose is not to rigidly answer every question, but to encourage players to ask deeper questions, and fill out the things that make their character interesting and robust, instead of just another Mary Sue.
1) Why are you adventuring? What drives you forward? What makes you crazy or desperate enough to do this to yourself?
2) What is something that you absolutely won't do, no matter what, even if it would cost you everything?
3) What is something that makes you feel truly alive? Something that you long for. Something that you would drop everything for?
4) What are two things about yourself (one good, one bad) that you are sure of, that nobody could convince you of otherwise?
5) What are two things about the world (one good, one bad) that you are sure of, that nobody could convince you of otherwise?
6) What are two things that you secretly believe (one good, one bad) that nobody could convince you of otherwise, but that you aren't even consciously aware of?
7) What is a situation that triggers you. A sore point that sets you off?
8) What is something that pacifies you, or makes you pause, even in extreme situations?
9) What is the first thing that people notice about you?
10) What is something admirable about you?
11) What is something contemptible about you?
12) What is something that you find aesthetically pleasing?
13) What is something that you find aesthetically disgusting?
14) What is something that you are irrationally afraid of?
15) What is something that you are irrationally fascinated by?
16) What is a hobby that you have?
17) What do you do when you are bored?
18) What do you do when you are nervous?
19) What is your favorite activity or game with others?
20) What is your favorite kind of humor/joke?
21) What kind of joke or comment would offend you?
22) Are you messy or neat?
23) Are you picky about your food?
24) Are you uneducated or well read?
25) How much attention do you put into your appearance and grooming?
26) What kind of slang do you use?
27) Do you have gaps in your knowledge, that would surprise somebody?
28) Are you more pragmatic, or superstitious?
29) Do you like to gossip? Do you brag a lot? Do you like to tell a good story?
30) How much do you lie or exaggerate to get your point across, or make something sound better, or make yourself sound more knowledgeable?