Interviewing your own clients is a suboptimal way to produce case studies. Instead, the interviewer should be a third-party who has never met your customer. Here's why:
1. You can’t ask fundamental questions.
Are you comfortable asking your best clients about what their business does, why they decided to work with your company, and how they use your product or service?
At this point in your relationship, you should already know the answers to all of these questions. Asking them now might feel awkward. So will asking your customers to tell you why you're so great.
But for a case study, you need these answers in the voice of your client. This context will form the the foundation of the piece.
2. Your client may not be real with you.
Your clients may be less inclined to give you complete answers when you already have a relationship with them. They may feel uncomfortable telling you the truth.
A third party interviewer can conduct a baggage-free conversation. They can bring back complimentary quotes to anchor your case study and insightful critiques that guide your company internally.
3. You know too much.
You understand your client’s business model. You speak in their jargon. Your client helped you craft your product roadmap. All of this is great for your business relationship - but not for your interviewing ability.
If you speak directly to your client, you’re both more likely to work from knowledge and assumptions that your audience won't share. You won’t approach the case study from an audience perspective. And when you use that jargon, you could drive readers away.
Ideally, your case study interviewer should be a proxy for your readers. Since your readers don’t have a prior relationship with your client, they are best represented by an interviewer who can enter the conversation fresh.