Everyone has his or her own style of writing and getting to know main characters. I’ve always found it easiest to design a character first and then plan a story around him. Some like to tackle the story first and let the character develop. Either way, here’s a list of details to consider as you build your characters.
Name and gender
Background - What are the characters’ race, culture, and nationality? What are their religious or spiritual beliefs? What was their upbringing like? What was their economic status? What relatives were present - parents, siblings, others?
Personality - What are the characters’ personality types? (There are several personality type scales and charts. I use the ones outlined in Florence Littauer’s Personality Plus book - choleric, phlegmatic, sanguine, and melancholy.) What are the strengths and weaknesses? What are their interests and hobbies? What are the goals of each character? (That alone can set the storyline.)
Physical attributes - What is the basic body build of each character? Height, weight, eye color, hair color? What is each one’s intelligence level?
Misc. - What are the characters’ ages or birthdays? Who are their friends? Do they have a spouse, girl/boy friend, or children? What other details would help define the character?
Those basics can provide a solid foundation from which to build. Sometimes it takes a bit to coax the details from our minds - or our characters if we decide to interview them. But the results will be three-dimensional characters who are ready to go any direction you send them.
What other details are important to know about your characters?
I’m also posting at the Insecure Writers Support Group website on How to Present a Professional Appearance as a Self-Publisher.