A phone interview is far preferable to an email exchange. People are usually more inclined to expand on key points and to offer up interesting details if they can do it via conversation, rather than in writing.
If in-person is possible, that’s even better.
Provide questions in advance so they can mull them over before your call.
Questions for the client:
What challenge prompted you to explore this solution? (Fish for concrete details you can use to vividly illustrate the pain the problem causes.)
Had you worked with any other vendors and/or used internal resources to solve the problem before working with us?
Why did you choose us over other similar products or services?
What was the decision-making process like?
How did implementation work?
How has the product or service provided a solution?
How have you benefited?
Questions for your internal project leader/team:
What solution(s) was used?
What was your approach/strategy? What about the client’s issues pointed to this approach?
How involved was the client? Close collaboration or was the solution more turnkey? (Gets to level of client involvement, which prospects often want to know)
What informed the strategy? (to highlight internal resources’ expertise and experience)
What about the project was most successful?
What did you learn on the project? Key takeaways case study readers would appreciate?
