Spending the time before you begin writing really getting to know your characters will make your job later much easier.
Here are three of the exercises that I sometimes use when I get stuck.
(Or Myspace, or even Match.com)
The Family Tree
It may seem silly, but making a family tree for your character/characters DOES help. I've made many family trees in my time, and being able to visualize your characters’ families really does help solidify them in your mind. Just make sure they’re clear enough for your readers to understand (I've had this problem in earlier drafts, but think it’s finally sorted out, phew!). At one point, I actually had a family tree program downloaded to my computer so I could refer back to it. Here are some suggestions for downloads of Free Family Trees (I haven’t tried any of these so I can’t vouch for them!). When you make this family tree, it is helpful to list for each member on a branch, their full name (and married name if applicable) and their birth date (year included) and death date, if applicable. Sometimes I put other notes in there as well, dates big things happened in that character’s life (ie, wedding days, if they've passed, what the cause, etc).
Disclaimer. You may feel like a crazy person when doing this exercise. But you are not.
The series I am currently writing stars a brother/sister team. Last semester I sat down with my female protagonist and we had a chat (a 3,000 word chat). Now, when it comes to her, I already know the basics listed above, I have a good picture in my head. I know what her work ethic is, I know her family history, I know her personality, her mannerisms, her tone when she talks, what irritates her, so I was pretty confident that I could have a chat with her and it would be true to her character. When you really get in the groove during this exercise, you’ll find it immensely helpful. I started out by asking her to tell me about the events in the book from her point of view. As per her personality, she went off on many tangents, which lead us to other questions, and a regular conversation. I realized in this exercise that she felt left out in the book, that her brother was getting too much of the spot light and she felt she was coming off too whiny and just as a follower. So I actually went back and re-wrote the entire opening to give her more agency, and it came out much better than any previous draft, because the two characters now felt equal. If you have never written your character, asking the basics and letting the character speak for him/herself will help you to develop the characters voice. You can also ask questions that will help you make up your profile (the first exercise listed above) if you haven’t already done that. You’re doing this for yourself, not for an audience, so you can write whatever you want, for as long as you want.
I hope these exercises help round your characters out. Let me know how these work for you, and if you’re interested in additional exercises.