I was introduced to a technique by a writer friend intended to enforce and ease the process described so ably by Demeter
in the preceding writeup. It focuses on the 'fleshing out' part of character development. I should state at the outset that I'm not very good at it myself, but I've been forcing myself to try for a couple of longer projects- and while I can't tell you if it's made any improvement in my characters, it has made writing
them easier, which is worth it all on its own.
This isn't a 'do what I say' writeup, but it's a 'worked for me' writeup. Take which parts of it you think best.
Get a Moleskine notebook and a comfy pen. Or a Newton. Or a raggedy spiral notepad. Or a legal pad and portfolio. Doesn't matter. The key here is to procure a means of writing things down that is reasonably durable and that you can easily keep with you.
For each character, go through Demeter's first few steps. Give them a name. Get a basic idea of what they look like. Write it down. When you get to the section marked 'Now their background' take a break.
Write down a list of questions you would ask your character if you wanted to find out the types of things Demeter talks about above. Be sure to phrase them as questions. What is your favorite color? What would you change about yourself if you could? What is one thing you fear above others? Do you have any brothers or sisters? How many? What did your parents do? Make sure you have at least forty or fifty questions. There's no need to do this in one shot; the entire point of having an easily portable writing system is so that you can do this either in deliberate sessions or in snatched moments.
While it's possible to do this on a computer, and I've tried, I've found that (for me) it just doesn't work as well. I think it's because I type too quickly, or perhaps because typing is too mechanical; I get done typing what I want to say before I have a chance to think about it logically or emotionally, and some of the nuance is lost. Once I have a solid feel for my characters, typing is fine because they'll have all manner of things to say that I need typing's speed to keep up with! In the early stages, though, there's some part of the process of writing with a pen that forces me to internalize them more. Muscle memory, maybe. This may not be true for you; take it as you will. I do always end up transcribing my Moleskine onto the computer for ease of reference when writing later, though.
When you have a good solid list of questions to ask your character, then it's time to start working. Taking the questions one at a time, in whatever order you choose, write down answers to them as your character. Speak in their voice. Think of it as method acting. Address you-the-interviewer directly if you'd like. If the questions annoy you as a character, vent about it; if they remind you of anecdotes, tell the story. If they are about subjects that cause you pain, be terse. If there are significant non-verbal responses to questions, such as shuffling in your chair, or wincing, or looking away, write them down.
This will take a good long time. Trust me.
At the end of the process, however, you may find (as I have so far) that when the time comes to wonder what that character will do, or what they will say - and more important, how they do or say it - will become easier and easier. It may become almost automatic for you, as you write about them, to slip into their heads and respond to situations, making your job as a writer even easier as you hurry to keep up with what your characters are doing.
There's no guarantee this will work, but at the very least, it will give you practice in writing with different emphases and from different points of view. Once you've done a number of characters, you can 'check' your work by reading the various answers 'cold' - give yourself several days away from the project first - and testing to see how different the various characters' voices sound when you read them again. If they don't sound very different to you, think about how a reader may have difficulty keeping them straight. Try again; pick some hot-button questions and let yourself chew the scenery a little while answering them in character. Strive for difference. You might have to tone yourself down later, but better that than struggle for differences.