From my experience, if you want to make big discoveries, ones that lead to deeper insights into your customers' pain, and where possible innovative features and solutions come from, you need to ask open-ended questions.
Don't ask overly specific questions or else their answers will be within the frame of what you basically already know which is good for making small and incremental decisions but limits your ability to find differentiating features.
So the next time you are talking to a client or prospect, ask an open-ended question than listen. Don't think of your next question. Listen. Then take a part of their answer that is most profound and ask why that is to dig deeper into their answer and continue to probe into subsequent answers asking further 'why' questions. Somewhere deep in one of their answers is their true pain and the possible genesis of a great product idea.