Many people play RP games as an escape - to immerse oneself into a fantasy
world as one of its inhabitants is a lot of fun and relieves the tedium of
day-to-day life. Some people choose to play by creating characters with
specific abilities and then mowing down everything in their path while
spending time with friends online and chatting.  For others, role-playing
games offer the chance to get to know their characters.  This can be done
either by developing a character over time and seeing in which direction he
"grows" or by giving him a complete personality before ever stepping into a
game's character creation! There is nothing wrong with either of these ways
of playing; both have successfully been accomplished.  But if you want to
roleplay your character and keep his personality consistent, a very useful
tool is a character bio sheet (aka development sheet). These are commonly
used by writers to get to know their own creations and to show publishers
that they have a handle on creating intriguing, well-rounded characters.  

Writing up a bio sheet is like interviewing your character.  You try to
answer a basic series of questions as best you can, about your character's
likes, dislikes, personal and/or family history, and other factors which
make your character who he is today.  The wonderful thing about a fantasy
world is that there are no real boundaries, so if you're not sure how to
answer some of the questions feel free to invent what you need!  They
really help cement who your character is and will help you to decide how
your character will react when placed into different situations.

SUGGESTED STARTING QUESTIONS:
1. What is your character's family like? How big is it? How did the
interaction he had with his family formed his personality?  

2. What is your character's first impression of others, and how is he
perceived by others?  Is he rude?  Polite?  Snobbish?  Educated? 
Simple-minded?  Serious?  Silly? 

3. What is your character's 'inner monologue?'  What are his hopes and
dreams?  His fears?  His insecurities? 

4. What are your character's motivations?  What does he want to get from
life?  One good policy to remember, in order to make your character more
believable, is to note that your character's motives must be consistent.  A
character should always act on his motives.  Only MAJOR events/happenings
that a character experiences should change him; these major events and the
resulting change should make sense and be obvious to all.
  
5. What does your character enjoy doing?  Does he have hobbies, such as
fishing, hunting, or reading?  Is he a scholar perhaps, his pleasures in
life coming from learning?  Or maybe he has led a privileged life and
prefers the pursuits that the rich indulge in?

6. What are your character's 'hot buttons?'  What makes him angry?  At what
point does his patience wear thin?  Also, how does he react when he becomes
angry?

7. How does your character feel about socially unacceptable activities,
such as murder or theft?  WHY does your character feel that way? 

8. How does your character relate to the gods, and why?
 
9. How does your character relate to people of different races, and why? 

10. What does your character think about other social classes?  And what
does he think about beings from other races, or of the opposite sex?  Both?

11. How does your character deal with stress?  Has he adopted any unusual
habits in dealing with stressful situations? 

12. How does your character communicate?  Does he tend to dominate the
conversation, or hold back, only interjecting when he feels it's important
to be heard?  Does he speak with an accent?  Does he use 'highbrow' manners
when speaking, or does he sound like a working-class character?

13. How does your character's race effect his daily life?  How is his race
treated in society, and does he fit the expected behavior patterns for his
race?

14. What kind of social position was your character born into?  What type
of culture?  How does this affect his life?  How does it affect how he
perceives others? 

15. What is your character's greatest joy?  Most humiliating moment? 

16. What is his most treasured possession and why?  

17.  How do your character's non-roleplay stats affect his personality? 
For example, if your character consistently keeps low intelligence scores,
how does that make him view the world?

These questions will offer a basic understanding of your character and will
help you make consistent, good decisions regarding how your character acts,
thinks and speaks.  The biggest thing to remember when creating your
character is to make it work for you - not all of these questions will be
necessary for you to answer for each character, and there may be some that
you want to ask of yourself that are not in this list. 

One good note:  you will want to know this complete background, but you do
not necessarily need to share it with others.  This is of course fine if
you wish to do so, but part of defining your character's personality is
defining how transparently he lives.  So if your character has a great fear
of spiders, for example, but also doesn't open up to others easily or
doesn't show emotions, he may not show a lot of reaction when presented
with a big hairy ugly spider.  But if he is outgoing and tells everyone his
life story, he might jump up, scream and run around the room.  The answers
lie with you and how you define your character. 

Good luck and enjoy!