I once heard a radio interview with a crime-fiction author (and no, despite racking my brains, I can't remember his name) that also covered the subject of believability. The interviewer asked how he came up with his plot lines, and he said that he had real-life members of the police force (active and retired) who helped him. They'd regularly get together, and would tell him stories about their working life, and from there he'd manipulate the 'real' events into fictional plot lines. He then went on to tell the interviewer that there were some things that would never make it into one of his books. Not because they were too gruesome or shocking, but because they would be deemed 'unbelievable' by the reading public.
That observation stopped me in my tracks. Real events that aren't believable? But... the fact that they're 'real' surely makes them automatically believable? Then I remembered the famous Mark Twain quote: Truth is stranger than fiction. Which I suppose, by default, means that real life is often stranger than anything you'll read in a book.
That's where 'The Case for Coincidence' comes in. Going back to those helpful articles; I've come across numerous instances where 'coincidence' is either deemed an inappropriate plot device (because who would believe it) or writers are advised to keep the coincidence relatively mild (for the same reason).
In other words: When turning REAL events into a piece of fiction, be prepared to cut out any wondrous elements in order to convince your readers that it could actually happen.
Are you seeing the irony here?
Take the case of Jim Lewis and Jim Springer. Two men who were twins, separated at birth, and who led independent lives until they were reunited at the age of 39. Both men had named their childhood pet dog 'Toy', they'd chosen careers in law enforcement, had been married twice (first to women called Linda and then to women named Betty), and both had a son they'd named James Allen/Alan. And there were even more coincidences within their lives than that...
So, could I, as a writer, get away with having characters with a similar level of coincidence tying them together? NO. Well, not unless I wanted to make it into a 'parallel universe' story. A story in which, for example, characters with lives like theirs were actually experiencing the points in time and space where parallel universes touch... that might make it believable.
The thing is, readers have certain expectations when reading a contemporary story, Real life only rarely holds such huge doses of coincidence. For those who haven't experienced it themselves, it's a reality that's hard to believe.
Which is rather a pity. And it means I STILL have to find fantastical ways in which to write about events that are, coincidentally, all too real...