In plotted fiction, it’s essential to get the lead character into some kind of predicament. And the predicament has to be serious. If there’s no predicament, you don’t have a plot at all, just a collection of random incidents. If the predicament is trivial or too easily resolved, the plot is a yawner. None of this is my original insight; you’ll find it in any number of how-to-write books.
Less often encountered, but just as important, is this bit of advice: You mustn’t let your lead character get into trouble by acting like an idiot. The lead must do the things that a normal, sensible person would do, given the situation in the story. If she fails to do the sensible thing, you must explain to the reader why she just can’t do it.
Let’s suppose there’s a kitten out at the end of the diving board. The kitten is in grave danger of plummeting into the swimming pool and drowning. Your lead character is darn well going to have to crawl out on the diving board and rescue the kitten! If she doesn’t, it can only be because of an overwhelming fear of heights, a phobia so severe that as a child she slept under the bed so she wouldn’t inadvertently roll over and fall out of bed while asleep. That is, her feelings (concern for the kitten) are normal and sensible, but her action is blocked by a stronger feeling, which you have carefully mentioned to the reader 25 pages earlier. If she has no compassion for the kitten, your readers will have no sympathy for her. As a lead character, she’ll be a dud.
I first noticed the idiot plot many years ago. By now I’ve forgotten what novel I was reading, but the story began with some people landing their spaceship on an unexplored planet, a planet teeming with life — and they immediately pop out of the ship and go for a hike in the jungle, without taking any weapons! Because what could possibly go wrong?
The characters in that story were idiots. I tossed the book aside without reading any further.
There are other ways to screw up the plot. For instance, your lead character can sit around passively while other characters solve her terrible problem for her. If memory serves, at the end of Lord Valentine’s Castle, a highly regarded book by Robert Silverberg, the hero finally makes it to the throne room of the castle, where the villain has been ensconced for the past few hundred pages — and the villain then obligingly commits suicide by jumping out the window. That’s not an idiot plot, but it’s a 100% fail. The hero has to solve the plot problem through his or her own effort. Allies may sometimes offer important bits of assistance, but the burden rests on the hero’s shoulders.