The first question to answer when developing a character is: Who is the character? Creating a superficial picture of the character in your mind aids in definition. What is the character's gender? What does the being look like? What is the character's profession? What does this character like and dislike? Once a sketch has been created, it is easier to place the character into conflict, and from there, the plot can develop.
The next question to ask yourself is: What does my character want? All characters must have some kind of desire that is driving them forward through the conflict. Take a look at the superficial list you just made. Why is your character unhappy and wants to change? For example, if your character is a lawyer, maybe she is sick of working for a corporate law firm and has decided to start volunteering with people in need of free legal assistance. What kind of new conflict could she experience?
Opposing forces and obstacles are incredibly important when developing a character. A great way to highlight aspects of your character is to mirror them in the antagonizing forces. Make the hero the opposite of the villain and all of their positive traits will be magnified. For example, if your hero is a lawyer helping poor people, make the villain greedy, power hungry and money-obsessed as a counterpoint. This strategy adds dimension in a discrete way.
Since conflict is integral to character development, it is important to define how the character responds to conflict. This dictates how the conflict is addressed. Does the character hide from conflict? Avoid it? Address it without truly understanding the facts? Characters respond to conflict with other characters in unique ways. For example, your main character fights differently against her boyfriend than she would her mother. Defining these relationships through conflict helps round out the character.