A PI story is a mystery which is solved by a paid investigator who doesn't work for a government agency. This includes private investigators, insurance investigators, and people like Lawrence Block's Matthew Scudder who acts like a PI but isn't licensed or always paid.
Traditionally, PIs are white knights who represent justice when the system fails. For whatever reason, the police and the legal system are unable to find or punish the guilty and so the PI is hired to set things right.
The private investigator is usually a fantasy character in the way that western gunslingers are fantasy characters. Most PI stories do not reflect the realities of being an actual private investigator. Instead, PIs fill the societal need for a hero.
PI stories are usually told first-person or with a tight third-person point of view. The PI carries the story and appears on every page without the luxury of scenes set elsewhere or narrated by other characters. To some extent, the PI is the story.
The novels of Sue Grafton and the stories in any of anthologies by the Private Eye Writers of America are examples of PI mysteries.
The PI is usually someone with a very strong moral center who pursues justice for justice itself as much as for the money.
PIs have historically been men since many states require a police or military background as part of the licensing requirements which limited opportunities for women. Now, of course, there are numerous notable female private investigators.
As popular female PIs written by female writers expanded the readership for PI stories, publishers sought other new marketable categories such as the Korean-American PI, the deaf PI, the beautician-by-day-PI-by-night PI. This trend has subsided somewhat. Perhaps all the niches have been filled or the fad simply passed. Write the story you want to write rather than the category you think the publisher is trying to acquire.
PIs are almost always loners although family (often estranged) does sometimes intrude to complicate matters. Most PIs work out of a one-person office (plus perhaps a secretary) but some are part of a larger agency or may supervise junior investigators.
As outsiders, PIs are often sarcastic or sassy, free to poke fun at bureaucracies, social classes, and hypocrites of all stripes. Many times they serve as an impartial judge and jury.
Private investigators (in fiction) often blur legal lines, especially when it comes to violence. The behavior of many fictional PIs would quickly land their real-life counterparts in prison but fulfills the expectations of many readers.
PIs often have a sidekick who can serve one or more of several purposes. The sidekick can act as sounding board, comic relief, or agent. As agent, the sidekick can do all the boring research jobs so that the PI doesn't have to bring the reader along or the sidekick can perform openly illegal activities without tarnishing the PI's image.
Since PIs don't have the access to information afforded to police officers, they often have contacts who work for motor vehicle registries, telephone companies, and credit agencies. These people, for friendship or money, provide details and background necessary to the investigation.
Most PIs work in a city since cities generate both enough crime and possible clients to keep a PI afloat financially.
Within a city, however, PIs may find themselves walking down dark alleys or through highrise corporate headquarters. Depending on the job, a PI should be able to blend in anywhere the investigation leads.
The classic PI storyline goes: 1) PI is hired, 2) PI stirs things up, and 3) PI picks up the pieces after the guilty party reacts in such a way as to prove guilt.
Today's editors seek variations on that theme. Too many stories have started with the PI sitting behind a desk interviewing a client. Too many stories have involved strong-arming all the suspects until someone overreacts. Too many stories have ended with a gunfight after the guilty person pulls a weapon.
Many bad PI stories have also ignored the realities of payment, contracts, and reports. PIs and their clients have a business relationship with all the legal ramifications that come with any customer situation.
PI stories often revolve around interviews and research. The PI is collecting information in order to recreate what happened or to learn where a person might be hiding. The reader sits on the PI's shoulder as this information is gathered and either matches wits with the PI or simply goes along for the ride.
Typically, the PI is thwarted by people unwilling to share information or lying to protect themselves. No one is obligated to talk to a private investigator who lacks the authority of a police officer.
Some PIs are leaving the loner image behind, coming fully equipped with family, friends, and outside interests. This is especially true with PIs who appear in a series since readers seem to like a community of characters. Other PIs are bending the other way, retreating into the darkness, walking a thin line between vigilante and criminal.
PIs can exist in small towns and communities if they also do other work. They can also travel widely if their jobs involve finding people on the run (and the client can afford the expense).
Subscribe to the mailing lists Rara Avis (http://www.miskatonic.org/rara-avis) and DetecToday (http://groups.yahoo.com/group/DetecToday).
Join or at least visit online the Private Eye Writers of America (no online presence but write Christine Matthews at 4342 Forest DeVille Dr #H, St. Louis, MO 63129 for membership information), Mystery Writers of America (http://www.mysterywriters.org), and Sisters in Crime (http://www.sistersincrime.org/).
Read the column by a working private investigator at Mysterical-E (http://www.mystericale.com/).
Read Writing the Private Eye Novel (edited by Robert J. Randisi) which is published by Writer's Digest Books.
Check the locality where you intend to base your character to learn who licenses private investigators and what requirements exist.
The following publications accept PI short stories. All but Detective Mystery Stories, Hardluck Stories, and Thrilling Detective pay. (Note: List was current at time of publication; links and details may have changed since.)