Open questions allow the participant to write down their own answer and are often seen as having more validity
Closed questions provide the participant with a selection of answers to choose from, which can seem less valid as it pre-determines the selection of answers
Give qualitative data, which is not numerical. This can make analysis very difficult but can provide depth and insight into an answers that the researcher might not otherwise get. It can also provide a push-point for further questions if needed
Give quantitative data, which is easier to analyse and spot patterns but sacrifices the level of detail and insight that you would get with open questions
The interviewer has a topic of interest they want to discuss with the interviewee
There is no set list of questions, instead it is an open conversation about the topic, which allows the interviewer to ask questions specific to the interviewee and gain further information