- A criminal case which contains a violent death. It should have its own special features which often hold the key to the mystery . It should be set in a normal setting, but contain unexpected elements.
- Excitement - create it in your style with simple uncluttered writing and short sentences, then raise questions or pose threats or reveal mysteries on every page and finally, create situations which require an immediate response from the main character.
- Characters - have a limited circle of suspects, each with motive, means and opportunity. Then a hero who is dependable, honest, lacking in self-confidence and slightly awkward. They must beat danger by being brave and persevering. Sidekicks should be dumb, and they must say what they are thinking. (Radice points out that Maigret is his own sidekick. He lies awake in bed asking himself obvious questions.) Then a villain - who will be interesting to the reader because criminals are active and free in spirit nor do they knuckle down under routine and a boss.
- Clues which add up to a tenable ending which the reader could have arrived at by a process of deduction. Clues can be hidden in lists, which readers always skim over, or you can use misdirection: go on about the great relationship a woman has with her daughter so we don't expect the man in the case to be her estranged son, or announce a false fact loudly, or by having lots of people assume that it's so.
And that's it! Probably there was more in the text if I'd known how to find it.