Useful article from Marketing Sherpa on ways of saving time writing case studies, simply interviewing customers. There are 7 steps in total:
Step #1. Choose customers to approach for a potential interview
Step #2. Invite customer to participate in the interview
Step #3. Write interview questions
Step #4. Record interview on conference line.
Step #5. Transcribe interviews
Step #6. Edit transcript
Step #7. Send interview to customer for final approval
Step #8. Promote some interviews publicly, but hold others in reserve
Here here