Interviews - The interview process

All interviews, whether structured or unstructured involve a social interaction between the interviewer and the respondent. The danger is that the respondent may be responding not to the questions but to the social situation in which they are interviewed.

Interviewer bias - This is where the interviewer may ask leading questions in order to prove a hypothesis, more than usual. Another source of bias is when the interviewer identifies too closely with the respondent.
Artificiality - Even the most relaxed unstructured interview is still an interview and as such, the conditions are artificial. It is doubtful in such circumstances that truthful answers can be obtained.
Cultural Differences - Different meanings may be given to different words. Or the culture gap may mean that the interviewer cannot tell when they are being lied to.
Status and Power Inequalities - In general, the bigger the status difference, the less valid the data.
Social Desirability - In interaction, people often seek to win approval. This may be even truer in an interview when respondents are on their best behaviour and give answers that present them in a favourable light. They may wish not to appear ignorant and may, if they don't understand the question, offer any answer rather than none at all. Unstructured interviews are more likely to have this effect.