But the writer also has a big “Why” to answer: Why their detective drawn to the case in the first place.
When I teach mystery writing workshops, it’s the issue I see the most often in beginners’ manuscripts. The writer has a detective, usually an amateur, who plunges into the case without any experience or knowledge of how crimes are actually solved, and from there, it’s a series of coincidences and luck that lead to a conclusion. But let’s be real—if any of us came across a dead body/broken bank vault/bloody, half-conscious victim, our first instinct would be to scream and call the police, not embark on a quest to put the perpetrator behind bars on our own.
Your detective can’t take it upon themselves to solve the crime simply because they’re the main character. They need a motivation, a reason to bypass the police and take the law into their own hands. You need to establish, for the reader, that this character is the only person who can close this case. Otherwise, you’ll leave the reader asking, “Why didn’t this person just call the police? That’s what we pay them for!”
In my experience as a reporter for a community weekly newspaper, this year alone I’ve covered three murders and a high-profile murder trial. All of them were committed by someone close to the victim and all of them ended in arrests within a few days of the crime, while the trial ended in a guilty verdict. Now, while that’s not always the case, your reader is going to assume that the cops, for the most part, know what they’re doing and have more tools to solve a crime than, say, a plucky hairdresser or a crafty dog-sitter. Not one of those three cases was solved by, say, a bright young journalist, much as I might like to take credit for justice.
But don’t send your detective packing just yet. There are plenty of ways to bypass the boys in blue, creating not only a more believable world, but adding layers of tension to your story as well. Perhaps the police arrested the wrong person and your detective now faces a ticking clock before the real burglar gets away. Maybe the murder looked like suicide or an accident, so the police aren’t investigating any further. Racism, sexism, or corruption go a long way too, and all you need to do is tune in the evening news to see real-life examples.
And because your detective isn’t bound by police protocol, that’s where you can have fun. She can dig through garbage, ask anyone anything without a lawyer present, etc. And because she doesn’t have access to high tech lab equipment, for the most part, she'll probably be asking questions--and she'll want to ask EVERYBODY. Shopkeepers, neighbors, delivery guys...all of them will have something to offer her to help her solve this case. He won't be dusting for fingerprints or checking ballistics, but your character can learn a lot just by talking to people...especially if, say, a character says she was out of town the night of the murder and the delivery guy remembers bringing her a pizza because she stiffed him on the tip.
Even if your detective is a professional—either a PI or a law enforcement officer—that motivation still needs to be present. No one wants to read a book about someone who’s just doing their job. Something has to compel your detective to devote all their energy to this case. Does the missing girl remind him of his little sister? Is the corpse an old friend who parted on bad terms? And does the detective resist the case? If so, what brings him back?
These are the questions you need to ask yourself before you put pen to paper. Because it’s about more than being the smartest character in the room. It’s about what pushes the character into a dangerous situation when they could be home watching TV and eating chips. Give your character that reason, and watch your whole story really open up.