another thing to doSo, you are starting. New feature to implement, complex modification, improvements in performance, user experience in UI or something completely different.
You got everything what you need to start, you got it from customer. Is he/her difficult with cooperation? Doesn't matter, you talked with him/her so many times, so many hours spent on learning his/her way of thinking, behavior. There is nothing unexpected in gathering requirements (or however you will call it). Even if it isn't easy job to do. You been there, you know what to do.