One of the more intriguing dynamics in startups and business in general, is customer communication. Customer Development is, of course, all about talking with customers to test fundamental business hypotheses, match product solution to customer problem, and in general, learn as much about them as possible in order more efficiently and effectively market and sell to them.
The tension comes from learning when to ignore your customers and when to take heed. Custdevguy reminds us that customers have their own agenda, which might not coincide with your own. Steve Blank reminds us that Customer Development is not just collecting web metrics and it's not about focus groups. I've written before that Customers own the pain, Founders own the vision, meaning that as an entrepreneur, you must tailor your vision to solve the customer's pain. That is the objective of speaking with your customers.
Sean Ellis perhaps says it best, describing the process as "honing in" on the "signal" that is the core value proposition of your product to your customer. What's valuable about this description to me, is that rather than looking at what you need to ask each customer, it provides a high-level perspective on what your objective should be and how to get there. It's talking to enough customer and asking whatever questions necessary to hone in on the core value of the product.