Who will be your sleuth

Amateurs, Semi-Pros, and Pros: Who will be your sleuth?

To a large extent, the type of crime you want to write about will determine whether your sleuth can be an amateur or must be a crime-professional. (And vice versa—the sort of story you envision may have sprung from your image of a certain sort of sleuth.)
Each variety of detective has its pros and cons, and it's a good idea to think about them as you get underway with your book.
The amateur sleuth's popularity is partially because he's an ordinary person called upon to do extraordinary work for which he isn't specially trained. This makes for easy and strong reader identification, because we want to believe that our innate intelligence, courage and (hitherto hidden) talents could, if tested, meet life's most challenging demands.
Writing an amateur sleuth also gives you the opportunity to include interesting locales or occupations, depending on your sleuth's address, "day job" and special expertise.
A plus for plotting is that the amateur sleuth isn't constrained by law the way private investigators and police professionals are. Within reason, your sleuth can do whatever works—assume disguises, lie, break and enter.
In general, because the sleuths here are ordinary civilians, there's less violence in stories featuring amateurs. These sleuths rely primarily on their brains—on what they understand or can deduce that others might ignore.
The downside of writing an amateur sleuth is justifying the need and ability to be a detective in the case. Even though your reader accepts the convention, you, the writer, still have to set it up so that your character feels compelled to enter unfamiliar and dangerous territory. For some reason (that you set up) the official investigation has to be headed in the wrong direction, possibly including the direction of your protagonist who may be the prime suspect—a springboard that can't be used in too many books in a series, however. Because of the crime and its handling, something intensely important to the sleuth is in danger unless he intervenes.
Unless you're writing an historical set before there were police departments, don't be oblivious to what the official investigation is doing. Research criminal investigative techniques and the law enough to provide a believable background and to avoid having your amateur appear ridiculous. (We'll discuss research in a later lesson.)
When thinking about your amateur sleuth, think ahead to the possibilities of a series and build in elements that might lead to further involvement in crime—an occupation that involves lots of contacts, a murky personal or family background, or whatever might help you build future stories around this amateur.
Somewhere between the amateurs and the professionals are the semi-pros—e.g., journalists, lawyers and insurance investigators who have logical reasons to be involved with crime but who aren't constrained by the rules regulating PIs and the police. The same rules of knowing what the officials are doing and giving these semi-pros real reasons to put themselves in danger apply, but you won't have to work as hard justifying why they're involved with crime in the first place. Just as with the amateur, however, this crime has to have some personal and urgent relevance.
The archetypal American crime hero is the private investigator, a macho urban cowboy prowling mean streets who is, according to Raymond Chandler, "...neither tarnished nor afraid." Today's P.I. is as likely to be a woman in the suburbs because the possibilities have expanded. However, the P.I. remains an inherently interesting figure- a seeker of justice who can bend the rules a bit and who is outside the system to a large extent.
Just be aware that laws governing who can be a PI vary from state to state—and the licensing procedure often involves time, an apprenticeship and regulations that must be followed if the results of the investigation will hold up in court. So again, do your research because even though this is fiction, and most real-life P.I.'s are seldom involved in the mayhem of their fictional cousins—your readers expect and deserve a realistic base beneath your imagined drama.
Playing by the rules—knowing how the system works—is still more important if you write a police procedural. One of the pleasures of reading such books is having an inside look at how the system does—or doesn't—work. One of the pleasures for the writer is not having to invent reasons for the characters to be involved with crimes and, because a police force is an ensemble, to develop a range of interesting characters.
But to state the semi-obvious, your police-protagonist, unless he is corrupt or incompetent, has to work within a code of behavior and even then, you need to know the code he's violating. So find out how your department is structured and who is able to do what.
Even when dealing with a crime professional, make this specific case special. Make it have some personal meaning to your sleuth. And then, whatever variety sleuth you choose can provide you and your readers a thoroughly enjoyable adventure in crime.