I was recently sitting in a small group writing session discussion on character motivation. The author leading the session had a number of great points, but the biggest and simplest was 'ask your characters why'. She went on to add, 'channel your inner 4-year-old child, and just keep asking--WHY?'
Little kids are trying to build their mental database as it were. If you tell them the sky is blue, they want to know why. And they will keep asking "why" to each and every answer until most parents just give up ;).
We need to do that with our characters to make sure there are real reasons for their actions and motivations. As readers, how many times have you read a book, or watched a TV show or movie and thought, "WTF? Why did they do THAT?!" Most likely the action or dialogue wasn't played through correctly and there was no valid character motivation behind it.
A very simplified example:
"Why do you race the other direction, way out of your way, to avoid firetrucks?"
"I don't like them."
"They freak me out."
"They're loud and scary."
"I'm afraid of fire."
"When I was five the house behind me burned down and there were little kids inside."
As the writer, you decide how much of the motivation to show. The character very well might not know the reasons for their actions, and it can and should be something that unfolds throughout the story. BUT-the writer needs to know it. (Back to the Iceberg issue a few posts back. ;))
A few rules:
1) "Because I am the writer, and I say so"- is not a valid answer. If you hit that, let the buzzer go off in your head and re-think what's going on. It may work as a last resort with your kids, character motivation not so much.
2) "Because I NEED them to do X-Y-Z"- BUZZ! Nope, see above, there needs to be a logical and consistent reason for the actions. Go back to square one and start asking your characters why again.
3) Some secondary characters really might not make this work. For example, this is what happened when I tried it with Crusty Bucket, one of the drunken faeries from The Glass Gargoyle:
"Why do you faeries drink so much?"
All conversation was lost at that point to her singing about dancing minkies.
Some characters just refuse to play. For minor ones, or deliberately difficult ones like Crusty, that might be fine. But if your primary characters refuse to get down to some serious levels of why? You have a problem that needs to be fixed. Yes, because I said so.