When you are developing characters it's important to a) figure out things that happened to the character before the story began, and b) figure out the things that happened before affect the character in the story. For example, Johnny grew up in the slums, had no father, and never had enough to eat. So in the story Johnny's tough, doesn't believe in the value of father figues, and enjoys a well cooked meal. When you figure out their back round it will help you determine how they will react to things, making it easier to write the story.
Another problem that I tend to run into (I'm sure other writers do too!) is when my characters are lose their spark. They're moseying though scenes with half-hearted responses, and out of character actions. I just lose who they are, and why they're doing what they're doing. If this has never happened to you, I hate to be a downer, but it probably will. Here's how to get those characters back into the game. Kill someone off. Minor characters are great for this. I know it sounds drastic, but it has a very good effect. When someone dies, people tend to examine their convictions, and purpose. It can also give characters a mission for justice, or revenge. Another thing a writer can do is have your characters ask hard questions of each other. Some questions can tie into their back round like "Why did you run away from home?", "How come you didn't tell me you were married?!". They can also be questions that deal with personality like "How come you hate France so much?", or "Why are you always so angry?" Whether you ask those questions for your, the writer's, personal understanding, or you can have other characters ask the question, either way it helps. To continue on with the "asking questions" concept, another tip is to use pre-formatted character sketches like this pdf, and questionnaires like the "Beautiful People" page I have on this blog.
The final thing that I've briefly touched on already is cause and effect. Characters just don't do things randomly. In real life people just don't decide to be "evil". Thats probably one of the biggest cause and effect problems in a beginner's writing. The antagonist can't just be evil because the hero(ine) needs someone to fight. He/she needs to have a reason, like revenge (cause), to kidnap the princess (effect), or they could honestly believe what they're doing is right (even though it's not). A strong piece of fiction needs to have cause and effect in its characters.
A summary: a character's past is key to current actions, killing characters off helps spur other characters into action, asking hard questions also does that, and cause and effect keeps characters in character. Hope you enjoyed this post, and it aided you in some way. I strongly encourage you to do character sketches, and also do the Beautiful People questions with me, as I've found both things very helpful. Anywho, this is getting much too long! Till next time!
EDIT: I just wanted to remind y'all about the features at the bottom of the home page, and the two separate pages at the top you can check out!