I've heard it said that the main characters will reflect at least some of the author's personality. I agree, although I think if one spends a lot of time with a main character, the author might begin to act like the character! :)
Occasionally you might decide to write about someone of the opposite gender than yourself. This can be challenging but interesting.
I think it's helpful to know your character's personality type because it will help you figure out what kind of choice they might make when faced with a certain decision. Their choice might be different from yours.
One way to help you think of other personalities is to think about your friends. Although my friends and I have much in common, we also have different personality traits. One of my friends is very intellectual and smart, she likes to talk about important issues but isn't really "into" hugs and feelings. Another friend comes in the room and people gravitate towards her, and everyone knows who she is. She's fun and sweet and make sure everyone is included.
God made us all with different personalities, so there's no reason your characters should have various personalities too. I think it's helpful to make sure you have multiple personalities presented in your book, because that's realistically the way it is in our world. Plus, many reader will be able to pick out a character that reminds them of themselves!
One thing I really love to do, I actually enjoy it, is to take personality quizzes pretending I am the character. You might want to take the quiz first for yourself, and then take it for one of the characters you know the best. The Big 5 Personality Test can be a huge help although I noticed that many of the tests are used for scientific research, so I would recommend not using those for your characters! Similar Minds has a good test, check out either the Short Test, Big 30 Test or the Big 45 Test for an in-depth look at your characters.
You can also check out some personality books, I'm reading one from a Christian perspective at the moment called Understanding Your Child's Personality by Dr. David Stoop. Yes I know, your characters are not your children...but it's still quite helpful!
What's YOUR character's personality?
-- Klara C.