In good fiction, everything is carefully chosen to exist. Everything has a purpose. Nothing accidentally fell into the story and wasn’t edited out without reason to keep it in. The golden rule is: if it’s not helping, it’s hurting.
Coincidences happen every day in real life. In fact, things happen against all odds, every second of every day. So why shouldn’t you have a coincidence happen in your book? You can, if you do it right, if it’s meaningful. But if you’re not carefully laying foundations for your plot then your characters end up as nothing but puppets dragged around by strings.
Successfully using a ‘call back’
This is a technique where your character casually mentions something, or something minor happens which fits the scene its in and bears no special meaning to the reader. In fact, the reader probably won’t even remember it was said, until you call back on it. Then that little seemingly innocent unimportant thing turns out to tie the story together. For example, your character casually mentions he dislikes rats when standing in a museum looking at a picture of one. Then 50 pages later on in the book, the only way to get the key to the safe is to retrieve it from a cage full of rats. He says he hates rats. He, and the reader, feel the weight of that moment 10 times stronger than it would have felt without that innocent unimportant line earlier in the book.
Don’t have any reference of him disliking rats before this grand moment? Revision, revision, revision. Just go back to that scene in the museum and add the dialogue line: ‘I hate rats.’
Failing to lay proper plot foundation
If, for example, you know you need a scene for your two lovebirds to meet up because information needs to be exchanged between them, you send them over the grocery store and boom! One shopping trolley accidentally hits the other shopping trolley, they look up and oh it’s you! What a coincidence! They start chatting and the necessary information is exchanged and they had another chance to flirt. Seems right, right? Wrong. If they have no real purpose in the grocery store then you shouldn’t suffer your readers through something as mundane as shopping. The characters aren’t driven by their motivations or by the plot.
Even light comedy should still be properly motivated. You need your character in a grocery store? Then he better be there to fight with someone over the last bottle of the perfect red wine because he needs to make the perfect dinner cause his unapproving mother is coming to visit and if he doesn’t make a good impression then he looses his trust fund.
Failure to provide back story motivation
Your two star-crossed childhood sweethearts find each other again by coincidentally moving in right next to the other! Imagine that! What a coincidence! No.. the reader doesn’t imagine that. This is where the reader disconnects from the illusion of reality you creates when you sucked him in and possibly where he puts the book down without picking it up again. It needs to be believable. Major events should not happen by coincidence.
Back story can help make this right. Motivation can make the incredible, believable. Why do they have to move into the same street? What’s preventing the author from just having one knock on the door of the other? What if it’s no coincidence at all that one moved in next door to the other, because he has actually hired a private detective to track her down after all these years because he always wondered what might have been?
The golden formula
Plot is cause and effect. It is a string of related events that happen in a particular way for particular reasons. They are never entirely unrelated events. They are the sum of characters lives and experiences. Each scene is meaningful and changes the character or the plot in some meaningful way.
The basic outline of any plot can be boiled down to his:
One upon a time, something happened (inciting incident), causing someone to pursue a goal (the prize). So he came up with a plan (strategy), but people with conflicting goals are in his way (opposition, conflict). There is a lot at stake, so he keeps moving forward (the why). Other people keep moving forward as well, increasing the obstacles in his way (the darkest moment). He learns an important lesson (revelation) that satisfies a need (satisfy the why) created by something in his past. Now he can obtain the goal.
Dixon, D. (1996) GMC: Goal, Motivation, and Conflict.