What makes a mystery? The three main characters help: The victim, the protagonist, and the villain.
The victim can be a nice person who didn’t deserve to get murdered, or a vicious schemer that had folks lining up to get a crack at him. What’s important from a plot standpoint is that the victim has lived their life so that they die NOW, at this particular place and time, and while in contact with a particular group of people.
The protagonist, or detective – be they a cop, private investigator, or amateur –
must have a strong interest in solving this crime. A police officer would have a strong professional interest. A PI would have both a personal and a professional interest in solving the crime – the professional because they’ve been hired; and personal because – as the story progresses – they begin to care about avenging the victim or feel a strong personal responsibility to the client. An amateur would probably always be personal – to avenge someone they cared for, or to clear their own name or the name of a loved one. If the protagonist is given a strong motivation to solve the case, this helps move the plot forward because it keeps the protagonist moving forward.
And the whole reason for the story: the murderer. There are all sorts of killers, but in fiction we writers like to stick with the tried and true: a serial killer, a murder for gain (money or love), or someone who thinks they have no other choice. This is my personal favorite and I find it most interesting. The person who commits the crime has been driven to this point by circumstances so horrendous that they thought murder was the only solution to their problem.
What would motivate a person to be murdered? Or to murder? What are the forces that drive a person? Is it money, love, security, or, most likely, a combination of them all? How would this person react if they were involved in a mystery? Would they be an active participant, in either detection or deceit, or would they attempt to extricate themselves from the situation? Is this a violet person or a passive person? What are this person’s interests and what do they tell us about the character? What is their physical appearance and what does that tell us about the character?
Agatha Christie may have thought of the peculiarities of a twisty plot, but to make it work she had to people it w/ characters that could live in that plot. Example: MURDER ON THE ORIENT EXPRESS. I’ve no doubt she thought of the clever twist as to who committed the murder before she thought of the characters on that train, but once she decided on that plot, she had to fill the Orient Express with characters who were capable of living out that plot and making it as believable as possible. Dame Agatha was a brilliant plotter, but she concentrated more on twists designed to shock a reader than she did on twists that emerged from the interactions of characters. Today’s plots are centered more on the interactions of characters rather than dependent on a cleaver means of killing a victim.
In my own books, character has a lot to do w/ the plot. Milt Kovak is a small town sheriff in Oklahoma, in a town he’s lived in all his life. He knows just about everybody in town. In most cases he knows the victim, and eventually, the murderer. The plot usually centers on the murder itself – as in a police procedural – but with lots of detours involving Milt’s many side characters – his staff at the sheriff’s department, his wife and son, his sister, and whatever else seems to be happening in Prophesy County, Oklahoma.
My E.J. Pugh series is more traditional, or cozy if you will. E.J. is an amateur sleuth whose first experience (ONE, TWO, WHAT DID DADDY DO?) is gruesomely personal. Actually, all the books have a personal interest for E.J., and many of them stem from something in my own family's life – not that we've experienced any murders, but, hey, what if?
In a traditional mystery there is usually a strong link in life between the killer and the victim. This immediately advances some of the plot: What were the circumstances that led to the killer’s decision to take a life? Was it an easy decision, a spur of the moment decision, or an idea that went terribly wrong?
In a mystery, the plot is the story. But it must ring true. Sometimes it's hard for an amateur sleuth to continually stumble over dead bodies and make that ring true, but there are other things in that story that should – the amateur's reasons for investigating, their knowledge of the victim, and their feelings about it. The truth is what matters in any story, and there should always be a nugget that our readers can take away.