Pitfalls when writing fiction

Many say they want to write a novel because the story has been going around their heads for years. Or some say they write for themselves, what they would want to read, or, as in my case, to have people forget their busy lives for a few hours.

Many sit down and write. A good thing, I believe, as long as they use common sense when making up their plot. There's nothing worse than a book full of plot holes or characters acting as if they were on horse tranquilisers, brain temporarily suspended. Or things happening so unreal it reads more like a slapstick comedy. The discerned reader will be quickly annoyed and discard the book as 'remind me not to buy another book by this author'. And rightly so. If an author expects us to believe his story, he must ensure that the story is believable.

For example: an murder in a crime novel is not investigated. Police arrives and merely state the person is dead, then one officer gives the eye witness a business card with the words, "We need to go, if you remember anything please give me a call." Worse, if he starts to flirt with the eye-witness, who, of course, smiles broadly at him, thinking of how she wants to kiss him.
Yes, I'd buy that. She just saw a murder and is totally calm as if she'd just baked a cake.
Who would write such nonsense? you probably wonder. Well, trust me, there are enough who'd do that.
I'm not a crime writer, but common sense tells me the police always asks tons of questions, they will have photographs taken, forensics team around, spending hours at a scene. A departure after 30 minutes is highly unlikely. They can flirt later, she can be asked to the station for further paperwork, or whatever reason (believable ones!) a week later and then she can think how hot he is. Fine.

Another example: a main character is friends with an arrogant stuck-up bitch who treats her like dirt, flirting with the guys she likes, telling her how fat she is and that she must lose weight, sleeping with her boyfriend, whatever nasty thing. There's no reason given as to why she would stay friends with her. No person in their right mind would be friends with such a person in real life. To create an unlikable character, yes, fine, but for them to be friends there must be given a reason like: they have known each other since childhood, the friend has just lost her parents the the character feels sorry for her, the friend has some sort of purpose for her, more popularity, has a car or maybe the character's secretly in love with her for some sick reason. It's important to explain to the reader the why.

In some genres you have to suspend your beliefs and that's okay, you know that there are no spaceships with whole colonies, you know there are no demons coming from the underworld, appearing in a blink of an eye, throwing fireballs and you know there are no fire spitting dragons, but even then, or better especially then, you have to have a well though-out storyline to make the reader believe it is really happening. And since many authors say they've been thinking about the story for years, there's no excuse for a sloppy developed novel.