Police interviews look very cool on TV, what with the traditional good cop/bad cop routine, but in real life they are a far more serious attempt at behavioural psychology. The reality is that few criminals actually confess during a police interview, and any information that is retrieved is useless without evidence to back it up. For this reason, the essential element that drives a police interview is to get at information that remains unknown due to lack of evidence. Most techniques involved in the interview process are therefore directed toward drawing out this unknown information.
The overall tone of an effective police interview is one of accusation. The accusatory nature of the interview need not be direct; it can be expressed in the overall tone of the confrontation just as well as through individual questioning. The purpose of the accusatory nature is to create anxiety in any interview subject who is lying. Except in the rare case of true sociopaths, most people who are attempting to conceal information experience inner conflict, and police officers are trained to spot signs that this conflict is taking place.
One very effective police interview technique is to ask a suspect what should happen to a person who committed the specific crime under investigation. The concept is based on the psychology of self-preservation. An innocent person is more likely to suggest that a guilty person be punished. The guilty person may instead suggest that instead of being punished the person should receive some kind of help, such as counselling, and suggest that the crime was committed because the suspect was sick rather than because of any inherent evil.
An important technique in the police interview is to check for the confidence of the suspect. Questions can be asked that reveal how the suspect expects things will turn out for him personally. Innocent people tend to express far more confidence that they will be exonerated than guilty people. This lack of confidence may not show up as an explicit difference from innocent suspects, but police officers are trained to locate subtle cues that reveal nervousness about the outcome of the investigation.
In keeping with the accusatory nature of such interviews, police will save a direct question about committing a crime for later in the interview. A sudden question such as "Did you commit that murder?" is far more likely to upend the guilty than the innocent. An innocent almost always answer very quickly and directly to the negative. The guilty person is far more likely to respond evasively at first with a response such as "Are you accusing me of committing murder?" The evasiveness of the response may vary, but what the police are really looking for is the lack of a quick and sincere denial.
It is important for the police officer to appear in control and confident at all times. Many police officers find it quite useful to appear to know more than they are letting on. The less confident that a suspect is about how much the police know, the more likely they are to make a mistake by saying too much or contradicting themselves.