Here is a template, in the form of a set of questions, for understanding a character when you first meet him/her. At the bottom I’ve linked to the sources for a few of the questions and references to some resources on this topic.
Age: How old is he? (And how old is he mentally? Is he a 40 year old in the body of a sixteen year old, or vice versa?)
Birthday: Day/Month? Year or age-as-of year?
Physical description: What does he look like? What distinguishing physical traits does he have?
Present situation (where he lives, how he makes a living, his social condition):
Childhood/history/future: Did he have a happy childhood? Why/why not? OR What will he be like in 20 years?
Past/ present relationships? How did they affect her?
Life’s “defining moment”?
What is the character’s archetype (see 4+ below)?
What does he care about?
What is he obsessed with?
What’s missing (physical, psychological, social)?
What MUST he do? What does he THINK he MUST do?
What makes him courageous?
What is his greatest strength? Weakness? Flaws?
What is the best thing that ever happened to her? The worst?
Most embarrasing thing that ever happened to her?
Does he have any grudges?
How does the presence of other characters or group situations affect him?
What is the one word you would use to define her?
What is his religious life like?
What are his idosyncracies?
How does he show/react to:
NOTE: find ways to make reactions surprising yet logical. For example: a character who is slightly claustrophobic might feel trapped living in a remote, country setting. Why? Because there is no immediately attainable safety net, the character sees no escape if there is some kind of urgent need for medical care or water or whatever. Thus the character feels trapped even out in the wide open space.
Some articles on this topic: