This site brings you the most useful resources on preparing and conducting key informant interviews. Let us know if you think that an important resource is missing.
Qualitative Research for Development: A guide for practitioners
2015, Practical Action
The publication provides useful guidance on several qualitative research methods, including semi-structured interviews, focus group interviews, observations, and various participatory methods. The document is relatively long but you can easily focus just on the topic you are interested in.