In most cases the customer is not aware of what else can be done. They've never had to describe what they need in a way that makes it unambiguous for us. In their minds, it is clear. Even the fact that they are thinking about converting user input to the number 1 is really going beyond the way they are used to thinking.
That's really as it should be. If they really new how to describe exactly what they wanted, they wouldn't need us to write it for them. As a result, our responsibility is to help them through the process. The process does require decisions to be made, so they also need our recommendations to make the decision process easier.
So let the customer be vague and talk at a high level. They know their business, and that's what they are good at (hopefully, or they won't be able to pay your bills...). Take what they talked about and think on it for a while. Eventually you get some great ideas to get them what they want and need, while ensuring that what you need is testable and consistent.
I highly recommend working in chunks. When you meet with the client have a set of requirements that are related to each other, and then explain how you intend to do what they want. Also explain why you made the choices you did. The customer can then look at what you provided and fine tune it. If you get a response like, "I never thought of that, but that would really help" you know you've got a pulse on how the client thinks. NOTE: that this isn't featuritis, it's selecting the right features to best fit the business problem that the client has.
If you have anything that looks like it might contradict what the client explicitly told you, then it's time to explain why. You'll need to bring out some issues the client never thought of, and how your alternative still gives them what they want/need but also avoids those potential issues. You may get a little pushback, but it also builds up customer trust as they realize you are trying to give them a product that they can really use. If they give some pushback, it forces them to expound on why they wanted something a certain way. That helps you understand your client more, and tailor the requirements as necessary.
The fastest way to wear out your client is to ask all the little questions one after another. You want to plan and schedule a series of meetings to review your approach. As long as you own the technical requirements (what your team uses to build the product) and your client owns the business requirements, and you can relate them together, you have a way to bridge the gap.