It is easy to define a use case following your idea of the best-case scenario, but it is also important to consider how the real-world execution of a use case might deviate from that, which is what this guide helps to do.
Using Personas to Get Functional and User Interface Requirements
Personas are descriptions of hypothetical people who will interact with a solution. This guide focuses on using them as a tool to gather further requirements.
Use this guide to help you build and manage a strategy for solution testing, which serves as a final check that all requirements have been sufficiently addressed in the solution.