Why Did They Do That?
There isn’t much that ruins a good story faster than lazy writing. A couple of days ago Paul talked about the use of nonsensical plot devices to get out of tricky situations. But at least as common is the tendency for authors to solve all types of problems by making their characters do things that are uncharacteristic.
There’s more than one way to commit this particular writing sin, and it can be very easy to fall into the trap if you’re not careful.
Perhaps the most obvious examples of this occur in comic books, and it’s largely why they’ve developed such a reputation as juvenile. Heroes, and especially villains, battle each other simply because they’re heroes and villains. The bad guys generally have no real motives, unless the author made a token background to superficially explain their behaviour, and heroes are helpful generally because the author wanths them to be nice.
There’s also a strong tendency to use the character version of the deus ex machina where the protagonist suddenly overcomes their biggest flaw, just in time to save the day.
Just as with plot tricks, this type of device is a symptom of laziness. In this case it’s an author’s refusal to understand their own characters, or the basics of human emotion an motivation.
Your character must act in a believable way. If your protag jumps in front of a bullet they better be very self-sacrificing, or have another trait—love, reflex, greed, etc.—overshadow their own self-interest. But even more, your character’s actions must stem from their pasts, their flaws and their virtues. They must be victorious because their virtues outshine their flaws, and they must lose because their flaws win out.
But if they act the way they do, simply because it’s convenient for the story then you don’t care enough about your characters to get to know them. And if you don’t care, why should your reader.