Don't write stupid, unbelieveable stuff that will only irritate the reader. If your herione hears something upstairs, knows her psychopathic cousin who's on the loose and is out to get her, goes upstairs to investigate with not even a weapon upon her, this is going to irritate the reader. ***Don't do that!***
Authors must strip their manuscripts or all coincidences or contrivance, or make such instances work for the plot. When we plot a novel, we should think ahead to the scenes that the storyline will engender and that cry out to be written: confrontation between heroine and pyschotic, the climatic moment when the disadvantaged hero finally socks it to the wealthy villain who seems to hold all the cards. Too often, however, we make the mistake of twisting events and contriving situations to force those scenes to occur instead of letting them flow naturally from the storyline and characters. The novelist must decide when these scenes are totally inappropriate and cut them out, however painful the process, or make them convincing and plausible enough to fit smoothly into the basic framework.
Sound motivation, clearly delineated, can do much to get a writer out of a jam: It can turn contrived situations into scences that are inventive and exciting; can make scenes that could have seemed contrived ring true; explain a character's presense in your story; and make a book's whole premise believeable.
Nothing will make a prespective editor toss aside a manuscript faster than a premise that seems ludicrous or contrived... unless the writer takes the trouble to make the outre' situation convincing. Piling up lots of significant details can bolster a farfetched premise and provide motivation for the action of the individual characters. Related to contrivance are coincidence and convenience, when problems are solved too easily for the protagonist, as if the writer can think of no other way of getting his or her hero out of a predicament. Yet coincidences do happen in real life. The trick in writing fiction is to make them plausible, or to make them the very basis of spellbinding sequences or even a book's whole premise.
One might turn coincidence into irony. There are coincidences to apare in the storyline, but they actually propel the novel along, giving it twists and turns and many moments of top-notch suspense. It works in spite of being "farfetched", because the characters and their motivations are real and the details totally convincing. The author makes you believe that, yes, it could happen.
Sometimes a coincidence can add just that touch of drama that your story needs. If a detective happens to see the felon he's just been assigned to look for as he crosses the street, it might b e pretty boing. But if a weary detective takes the commuter train home because his car broke down (a perfectly believeable situation) and at the station sees the fugitive he's been hunting down for months, you have not only another believeable situation but an ironic and exciting one. It's coincidental, yes - and yet it's not. The detective saw his man only because his own routine had been altered. It could happen.
Because "ordinary" character in thrillers usually have to overcome the odds and ultimately triumph over their opponents, they often seem to succeed too easily. The solution to this problem is to have the protagonist seem less of a superman by making his antagonist flawed, uncertain, and human. But don't pull these flaws out of nowhere. Let your reader know in advance, for instance, that guards surrounding the adversary's fortress have been partying all night, so it won't seem farfetched later when the housewife-herione sneaks by them without being seen. Let your readers know that the evil dictator, vampire, or what-have-you occasionally undergoes seizures, so that it won't seem too convenient for him to have a seizure at the climactic moment and thus allow the hero to escape.
Don't make things seem too easy, often the more hopeless the odds, the more desperated the situation, the harder and more desparately your hero has to fight. He or she may triumph - convincingly - simply because there is more at stake. There is no need for a "convenient" solution. Thus is not to say that police or agents cannot arrive at some point as they might in real life, but they shouldn't show up - if at all - until the hero has had a pretty hard time of it, has almost used his own ingenuity to survive, up until that point, and is close to taking his very last breath.
Having your protagonist face "unimaginable horrors" or a truly overwhelming and petrifying antagonist, means that you must explain how he can carry on to the end, when in real life most people would collapse or go mad. This can be accomplished in several ways. Explain the character's ability to cope and continue their campaign to eradicate the threat, by showing how they summon up their courage, even as they're quaking with terror. They do what they do because they have to. If they give up, who will take their place?
How would you slove the problem of the terrified herioine who goes upstairs to investigate a footfall? It is dramatically right for the woman to face this psychotic relative on her own, without the boyfriend's assistance. Yet at the same time it is totally unbelieveable that she would do so.
Or is it? ere is how I would handle it, through motivation. The young woman has been hounded by her insane cousin for so long that she just can't take it anymore. Although she is terrified, her primary emotion as she goes up the stair is not fear but anger. Why should she cower downstairs waiting for him just because she's a woman? Though she doesn't have a gun, she can always get a knife from the kitchen. Maybe she does wait for the boyfriend to return, but as more and more time goes by, she's afraid something's happened to him, and feels that she has no choice. She has no car, and she is determined not to be driven from her home to tremble in the woods and dark. She decides to face up to her greatest fear.
By using this technique, you can not only make "contrived" scenes believeable, but convince the reader that it is the only way the scene could have been written, that what occurs is the only thing that could have happened. It is up to thw writer to suspend the reader's disbelief, to make sure the story flows smoothly, and that there a few, if any, moments when an editor stops and say's "Now, wait a minute!"