Hard-boiled detectives originated with pulp magazines and were first in circulation in the 1920s. These detectives were popular, especially in noir. The detectives often lived outside the law, made up their own rules, had a rough outlook on life, and usually had a tragic back story.

The key to remember is that the detective is a protagonist and should be likable, no matter how unlikable some of their personality traits are. They can have snark, drinking problems, a rude streak, or whatever flaws fit the character wanted, but they also need to have redeeming qualities that makes them worth the time to read. The detective needs to have several flaws, and can be a detestable person, but no matter how horrible they seem on the surface, they need to have a quality that makes them likable.

Most hard-boiled detectives have a tragic backstory, which is usually rooted in the loss of a loved one. The loss is usually part of a past case they were never able to solve. They go up against a killer that decides to exact revenge against the detective by killing the one they love most. Typically this tragedy occurred in the past, and often comes back to haunt them in the present. Somehow they still blame themselves for causing the death and still carry the loss around with them.

The detectives aren't always part of the police force, and sometimes are former cops turned private investigators or investigators from the start. Even in this instance, they still fit the archetypes of a hard-boiled detective. When the detective isn't part of the police force, they usually have a bantering or estranged relationship with the police.

The hard-boiled detective is usually hard to deal with and often unpleasant to be around. They're rough around the edges and don't take advice from anyone. They have a lot of attitude, but a strong need for justice, which they usually achieve by not following all the rules.

A lot of the time, these detectives are down on their luck. Maybe they're barely keeping their investigation business afloat, or are in fear of losing their job. This is a driving force for what makes them go after a big case that could turn everything around for them. The detectives usually take a personal interest in the cases.

The world of the hard-boiled detective isn't the upbeat and bright world of the cozies. They're dreary and serious, often dealing with a lot more serious topics in addition to murder.

The tone of society and the hard-boiled detectives' surroundings are often gritty and darker. It's usually a more gruesome environment, and there's always a murder at the heart of the investigation. When the detective gets involved, they're nearly killed and barely get away from the murderer alive. But when they solve the big case, it tends to save their career, at least temporarily. The stakes are high as the killer seeks out more victims and the detective is usually working with a limited time table.

The style is more procedural than a cozy mystery. This isn't an amateur sleuth that stumbles upon a mystery; this is a detective that solves crimes for a living. Additional characters include a medical examiner, crime scene investigators, witnesses, and suspects. The style is professional and requires research into how cops investigate, how evidence is gathered, witnesses are questioned, and suspects are interrogated.