Understanding and interviewing your users is a crucial part of delivering the right things. While there are many ways of doing this, there's a particular theory that I was encouraged to follow by my design professor that is still stuck with me: Contextual inquiry.
The contextual inquiry is not just about interviewing your users. It is built on the idea that systems design should be grounded in future users' work and aim to enrich the work through the new possibilities offered by information technology.
It is user interviews combined with observations. The goal is to construct a rich picture of the actual work situation: roles, responsibilities, problems with the work and existing tools, etc.
A few principles guide it:
1. Context: Start with the actual work, not with what people say they do
2. Partnership: Future users are experts in their work and should be jointly responsible for the inquiry.
3. Focus: Everybody focuses on something different in the same situation. Therefore it is essential to be aware of this fact and actively try to extend the total focus of the inquiry.
The interview is then best performed when the user is working on their daily tasks, and the interviewer is sitting next to them while they discuss what is happening and what that means. The interviewer should note what the user is saying and what they are actively doing. Especially if any issues arise with the tools and to what extend the tools are supporting the work.