In-depth interviews are similar to group discussions, but usually only one person (but sometimes two) is interviewed at a time.  The format remains unstructured, the interview is generally conducted by a researcher, and projective techniques may also be used.

In-depth interviews are ideal for investigating personal, sensitive, or confidential information which is unsuitable to cover in a group format. 

In-depth interviews are also valuable for researching people with busy lifestyles who would be unlikely to attend a focus group - e.g. senior business people or doctors, or simply for those who cannot practically attend, e.g. farmers or lone parents with young children.

In-depth interviews can vary by the objectives of the research.  The examples below show how these can vary:

Paired / Coupled interviews, which are useful for gaining an understanding of particular behaviours or decisions which consumers usually make in pairs, e.g. husband and wife interviews about home improvements or car buying.

Accompanied shopping / observations are when a researcher accompanies a respondent on a given activity, observing and questioning the respondent’s behaviour as they proceed.

Business interviews, which are ideal for interviewing senior and/or very busy business people, as the respondents can discuss commercially sensitive information; be interviewed at their most convenient times and places; and be given the level of respect they may be accustomed to.