You need detailed data that explores people's emotions, experiences and feelings
You think it is reliable to collect data from a relatively small number of informants
You want sensitive or personal information. Interviewees are more likely to share this information when talking to the researcher rather than filling in a survey.
You have easy access to relevant interviewees. Interviews are costly and time consuming if you for instance have to travel far in order to find interviewees and get a good spread on your research.

Structured interviews are rather similar to a questionnaire except the researcher is sitting face to face with the respondent. The questions in a structured interview are carefully composed and every interviewee is faced with identical questions.

In a semi-structured interview the researcher has a clear vision of what area to investigate but the interview is more flexible for example of the order the questions are asked. In the semi-structured interview it is more important to let the respondent develop his own thoughts and the questions in his own way.

In an unstructured interview there is even more focus on the thought of the respondent. Here it is up to the researcher to inspire the interviewee to share their opinions and views on a certain theme or topic. The researcher role is basically to get the ball rolling in the right direction and then document the respondent's train of thought.

When conducting an interview the researcher should try to minimize his or hers effect on the outcome of the interview by taking a neutral and passive stance. The researcher should therefore strive to present himself in a neutral appearance (clothes, hairdo, courtesy etc) and stay non-committed and neutral to the respondent's answers during the interview.

When recording the interview the researcher has three methods to choose from; field notes, tape recording and video recording. Field notes can be done under or directly after the interview and will be the researchers interpretation of what was said during the interview. A tape recording on the other hand will catch the exact words and phrases from the interview but will not record non verbal communications. A video recording offers the most complete recording when it catches verbal, non-verbal and contextual information but is on the other hand the most intrusive and bulky of recording equipment. When using tape and video recorder one should always stock up with extra batteries and cassettes.

One-on-one or face-to-face
This is the most straightforward and common way to conduct interviews. An advantage is that the information comes from one source and that the interviewee can't avoid or "hide" from the questions in the same way as in a group interview.

Telephone interview
Essentially the same as a "normal" face-to-face interview but conducted over the telephone. This allows you to reach further in a shorter period of time or when you don't have the resources to travel. The drawback is the loss of nuances that occurs when you can't see the person and pick up on subtle signals. variations on a non face-to-face interview techniques may include; video conference link or a text based chat room or messaging tool.

Focus groups/Group Interviews
Focus groups: Group interviews normally consist of a group of 4-6 respondents. The group interview is a bit trickier to plan when many peoples schedules have to match and therefore often take more time arrange. A group interview often generates rich responses when the respondents can challenge each others views and discuss the topic within the group. On the other hand in group interviews the opinion for the reserved and timid ones can disappear when the discussions to a grater extent are

On the street
A structured or semi-structured interview technique that is similar to a survey where you ask people in public a number of questions and make recordings. The sample is usually random but could depend on the environment or other factors such as only asking couples, tall people, etc.