I was having a chat to a writing friend of mine, and she asked me "What is a weakness (in a fictional character)?" Now, to me, a weakness is "a trait belonging to a character that will ultimately destroy them, or make it impossible for them to reach the end goal in the tale." "What is it then if it doesn't destroy them?" Then, I replied, it's a Flaw. I got the equivalent of a confused look from her. A Flaw is "a trait that either creates an obstacle, or creates a diversion, for the character that makes achieving the end goal more difficult."
Now, before you go scrambling for reference materials on writing, these terms are mine. This is how I see these traits. Now, something that could be a flaw, or a weakness, can actually be a strength or benefit to the character. Ambition is one clear example. It can be that one thing that keeps the character going forward when nothing else does. It could be the anchor around their neck, holding them back until they come to the realization that it's not good for them, and they cast it aside, or it could be that one thing that causes them to not get where they wanted to go because they were too focused on the ambition, rather than the reason for it.
My friend said, "There are no perfect characters, and if there were, they would be damn boring." And it is true. Half the fun in character development is finding the protagonist's flaws, and seeing how they can be (ab)used in the story to make the tale more interesting. Flaws can be anything. Nervousness when talking to a group of people more than three or four. An allergy to cats. Fear of heights. Indiana Jones' fear of snakes. The list is endless.
But the flaw is there to be used. To have an impact on the character when they don't really want it or need it. The protagonist has a fear of heights, and is chasing after the villain who possesses something important to the protagonist. Yet, the chase leads them up into a construction site, and along a scaffolding bridge over a rather long drop. The protagonist will have to deal with his fears, while trying not to let the villain get away. The protagonist may not be successful, which would lead to complications further on that increase plot tension. Or, the fear may be overcome, and get a much needed win to spark the dwindling hope. But, if it is mentioned, then it is best that it be used. The same rule applies for any strengths a character may have. Demonstrate that it is there before it is used at the crucial moment.
But flaws are great plot hooks and can be used in many different ways if you are creative enough. Of course, some flaws are better suited to that than others. Harry Dresden from the Jim Butcher series "The Dresden Files" has a perfect flaw for this. Harry can't say no to a woman in need. Deep, deep down, there is an unstoppable urge for him to help the damsel in distress. That flaw gets some serious mileage, but how it manifests turns out rather different every time.
But give your protagonists flaws, things to make their lives more challenging within the story. Keep them realistic and consistent. Don't have the limp disappear when they need to escape a bad guy, that's just bad form.
