Recently, I read some books of a series which I very nearly found intolerable due to the characters' complete inability to be effective. Massive numbers of plot points rested heavily on the characters' failure to act. This was not limited to the main characters - in fact, I came to accept the series mainly by deciding to regard it as taking place in a fantasy world wherein one of the rules was that no character may be effective.
This is not to say that characters should be good at everything. The main point I'm trying to make is simply that characters should do what they can, especially in serious scenarios. In the aforementioned series, characters would stand around, not taking the obviously beneficial course of action right in front of them, in life-or-death situations. The resultant feel of the characters was somewhere between extreme hopelessness (they just figured it would never work) and extreme stupidity (it didn't occur to them).
There are a number of in-story reasons why characters do not take appropriate action to their circumstances, but many are thin disguises for the fact that the plot needed it. Good reasons stem from character: He has an established fear of water, so he cannot cross the river to safety. She was taught to see animals as people, so she won't eat meat until the point of starvation. And so on. In some stories, however, we get things like the following (example completely made up; not taken from any story of which I am aware):
"Edward suddenly understood the cook's threat: his father's tea must be poisoned.
'Father!' he cried. 'Don't drink the tea!'
'Don't be silly,' said his father, raising the cup.
'No!' Edward watched helplessly as his father took a sip."
Now, who can think of how dear Edward could have been more effective? He never said "The tea is poisoned," and made no attempt to physically stop his father from drinking it. And frankly, anytime a word like "helpless" is used, there had better be a good reason for the character to be helpless. The main issues here are believability and whether or not the reader can - and wants to - identify with someone so useless. (Though even non-protagonist characters, who are not burdened with providing the reader a guide through the story, should act as appropriate to their situations.) Here, chances are that the imaginary author's imaginary plotline involves Edward finding an antidote, or witnessing his father's death, or finding out that his family's male line carries a bizarre biological immunity to poisons, or some such. If he prevented his father from drinking the poisoned tea, the plot would not progress. However, Edward's motivation should not be "to advance the plot," but "to save Father." He should put appropriate resources toward any goal based on its seriousness - thus, in this case, all his resources.
This can be a particular problem in stories with young protagonists and aimed at young readers: these sometimes seem to imply that the characters could not act effectively because they are children. Not only does this insult young readers, it gives them a solid vote of "no confidence." Besides that, it is unrealistic. A child character may not apply CPR when someone falls, but she can certainly go for help.
Most readers do not expect characters to act as they would in those circumstances, but they expect the characters to act as they would if they were those characters under those circumstances. One person might think "Boy, if my father was about to drink poisoned tea, I would use my ninja skills to shatter the cup with a shuriken." Still, she would probably not expect this of Edward.
I thought these points were obvious, but they seem not to have occurred to a number of authors at all. At any rate, this may be a quasi-rant, but if I can make one person think twice about writing inexplicably helpless characters, I will call it a job well done.