Interviewing victims, witnesses and suspects is an integral part of a police officer's duties. Whether taking a report of a missing person, writing out a statement from a witness to a car accident or investigating a homicide, police must talk to people in a manner designed to elicit useful information. Police use various interview techniques depending on the subject matter and characteristics of the person interviewed.
The International Foundation for Protective Officers' 2005 report on investigative interviewing defines an interview as "a conversation intended to elicit information." Different types of police interviews include receiving a general complaint; receiving a victim report; elucidating a witness report; and investigative witness interviews. Different procedures are used to interview children or other vulnerable victims.
A domain interview is intended to find areas of interest to pursue later with specific questions. Police use domain interview techniques when a person comes to an officer to report a problem or offer information, but the officer does not yet know the nature or scope of the problem or areas in which information is being offered. Police will ask broad, open-ended questions in an attempt to ascertain what information to pursue with the interview subject.
A structured interview is one in which the interviewer has prepared specific questions in advance. Police use structured interviews for initial intake contacts to determine which officer or department can best address a person's situation. Police might also use structured interviews for routine tasks in which known sets of information or data must be gathered, such as in a car accident report form. Most interviews start with a short structured segment for gathering basic identification information.
In unstructured interviews, police ask open-ended questions designed to encourage individuals to tell their own story. Police use unstructured interviews in investigations to learn more about the incident as well as to assess the credibility and character of the person telling the story. Unstructured interviews are likely to lead in directions the officer could not have anticipated in preparing a structured interview.
Interrogations are a type of interview in which police attempt to obtain a confession, or incriminating evidence, from a crime suspect. As major crime investigative interviews proceed and the range of suspects narrows, the line between interviews and interrogations might blur as police attempt to correctly distinguish criminally culpable and non-culpable persons. Police interrogations are accusatory by nature, and police must advise the suspect of certain legal rights before proceeding with interrogation if the suspect is in custody.
Behavioural techniques can be used by police in interviews and interrogations to determine whether the person interviewed is telling the truth. Behavioural interviewing means asking questions designed to provoke behavioural responses such as fidgeting, stammering or evasion. The theory behind behavioural interviewing is that if a person is lying, he will perceive such questions as accusatory and respond defensively or become flustered by the effort required to fabricate a response.
A 2006 study by Professor Aldert Vrij from the University of Portsmouth, England, determined that fidgeting, avoiding eye contact and other behavioural responses to interview questions were not good indicators of lying. This study determined that verbal, rather than behavioural, responses were the most accurate indicators of truth or deceit. Vrij's team also determined that imposing mental stress, such as requesting the subject tell their story in backward chronology, made maintenance of a lie so difficult that lack of veracity became obvious to interviewers. Many police have now adopted mental stress interview and interrogation techniques.