Decide on the goal

Are you after gaining general customers insights on a domain/topic or are you looking for something more specific? Such as chances of a customer taking a new service into use, improvement expectations for an existing service-in use etc.. It is important to clarify the goal of the interviews because the interview questions would be based on that. For example in case of the first one (general domain exploration), an open ended interview with couple of flexible loose questions to create a flow of converstion would be enough.

Set up an interview team

Ideally at each interview there should be 2 of you as interviewers. You can have a bigger team of interviewers rotating between each interview but having one same interviewer present in all the interviews would be helpful to be able to cross-analyse the full of set of interviews.

The interviewers should be knowledgable about the topic but they shouldn't be opinionated or at least be able distance themselves from the service/product in question so that people can feel comfortable with giving their honest opinions. If you say that you are the inventor of the idea and try to get people's honest opinions about that idea, you will most likely fail since no one would want to "call your baby ugly".

Reach to right people to interview

This is a critical point and you should take your time to do this right. You need a variety of customer types ( dislikers, fans, neutrals, decision makers, influencers) and stakeholders, ideally invested in to answer your questions as best as they can. Interviews shorter than 30 minutes might fall short of building a rapport and diving deep in to specific topics. So it's best to schedule for 30-45 min. intervies. Scheduling all the interviews for the same week would help to build a momentum, but still giving you time to pause and improve if needed.

Create an interview script

Start with generating many questions by brainstorming within the team. Then cut it down to less than 10 questions. Questions should be open ended rather than yes/no questions and should be focusing on the customer behaviour and underlying reasons besides the customer opinions. Such as "How do you do this task today?" and following up with "Why?" questions. Please see the case study examples for more details.

Other practicalities Have your recording device, notebook, interview script print-outs ready. You might also need a Non Confidentiality Agreement signed in some cases. Make sure the interview room is a relaxing space and it might be a good idea to have a small gift (e.g. couple of movie tickets) presented to the interviewee in the end.

Interview (30-40 min)

After welcoming the interviewee and thanking him/her for their time you can start with the warm up questions from the script. Even though you worked on that script, you don't have to follow it step by step if the conversation takes you to more interesting discussions. You need to be an active listener and show genuine interest and be non judgmental about what your interviewee is talking about. S/he will notice this and be more willing to share their anxieties, prejudices and insecurities. Achieving this is your most important goal as an interviewer, not running through the script. To achieve this you need show your curiosity and ask for probing questions and examples in response to answers. Repeating back what you heard, and asking the question in a different way to verify it helps to clarify the answers. It's also important that the interview doesn't feel like an interrogation but rather a conversation. So you should also give chance for the interviewee to ask questions.

Towards the end of the interview observer/note taker could also have a short slot to ask questions. Wrap up the interview by thanking again and presenting the gift of appreciation if you arranged one.

Post-interview analysis (20-30 min)

This is another critical step to get the most out of the time spent on this exercise. As the interviewing pair you go through your opinions, feelings and notes and try to come up with a one-pager that summarizes the interiew for rest of the team. You can use a template for this so these summaries are petty much standardized accross interviews. But make sure you have a section to record the miscellanous notes and findings.