The difference between interviews and interrogations is striking, but they share a common purpose. Both of them are an attempt to extract information from another person, and they are both typically performed in a one-on-one or small group setting. The similarities typically end there, though.
For one, an interview is typically an agreed-upon discussion that is pleasant in nature and has a relatively positive goal in mind. An interrogation is the opposite: one party is trying to elicit information from a second, uncooperative party, and the motivation behind agreeing to reveal information is to prevent worse situations from befalling you.
Additionally, a secondary purpose behind interrogations is intimidation and a display of force. Interrogations serve a main purpose of gathering data, but they can also be useful to intimidate and frighten the second party to prevent them from doing anything against the first party's will.
In some ways, interviews and interrogations are rather similar. However, they are also quite different. An interview is much more of an open-ended attempt to gather information. By contrast, an interrogation is more of an attempt to get a person who is suspected of a crime to admit their guilt.
In one sense, interviews and interrogations are similar. They both consist of a police officer or officers talking to an individual. They are both, in a sense, efforts to get information. This means that some of the same techniques can be used by an interviewer or an interrogator. However, this is where the similarities end.
An interview is typically done in a relatively relaxed atmosphere. It will, for example, be conducted in an interview room rather than in a secure, lock-up facility. The level of security is lower in an interview. For example, police typically do not carry weapons when conducting interrogations lest the suspect take the weapon and use it on the officer. Police officers are not accusing the interviewee of having done anything so the tone of the questioning can be rather different. The police are not necessarily pushing an interviewee to give a specific piece of information, so the questioning can be less focused and certainly less accusatory.
Thus, these processes are similar in some ways. They are both attempts to elicit information and some of the same skills are needed for both processes. However, there are serious differences between the two in terms of their tone and the circumstances in which they occur.