Well defined leading characters can (almost) remove the need for a plot outline. Not that I'm advocating this, but it can certainly come in handy if you've managed to paint yourself into a corner as far as story development goes.
If your characters are well defined you should be able to ask yourdelf that one important question and the answer should be obvious:"What would he/she do in this situation?"
If you have sufficient backgroud for your character(s) and they have been well developed then this should get you out of even the most tricky situation. You may even find the story taking an unexpected, but interesting, direction.
If you can't answer that question then the chances are you need to put in a little more work on your characters.
So what is required for a "well-defined" character? You could read a number of articles and get a number of different answers but I think there are some basics that should be adhered to:
- Physical description, age, marital status, occupation, residential status (with parents, renting, homeowner etc.) and anything else you would find on a standard application form should be established first.
- Brief description of childhood. This should include where they grew up, what their parents did for a living, whether they were happy, what sort of school they went to, what their hobbies were.
- A list of current friends/partners with a brief description of where they met, how often they see each other. Of course some of these may also be primary characters.
- At least three memorable incidents from their life: winning something, a serious accident, a personal trauma, that sort of thing. The more that spring to mind the better. These are the things that define peoples attitudes to current life events.
These are just the basics and you'll find that once you've started a lot more information will come to mind. Write it all down and keep track of it, even if you think it seems irrelevant and would never matter to the story. You may be surprised.
Of course none of this information should be set in stone. If you reach a point in the story that would require your character acting in a way that does not fit their background then amend the background slightly so that it does. That car accident might become a near-drowning experience while sailing.
The important thing is that by defining the character in this way you are keeping track of who they are and you will know if you're trying to force them to do something they wouldn't.