In my experience, often it’s hard to find that person with deep domain knowledge, and even when they do, it’s unstructured, they take things for granted they shouldn’t* and the have no appreciation of the demands of formalism.

Getting anything you can use to construct a work plan, never mind a detailed feature list, out of clients can be a dark art.

*To the point I have repeatedly experienced a point close to the end of the project where they go “What do you mean you don’t handle a case I have failed to mention for the entire duration of the project?”