Personas
Part of thinking about the user story involves thinking about the user as a distinct person. What do they care about? What skills do they have? What are they trying to achieve, and what are they trying to avoid? Coming up with different personas as stand-ins for real users can be very useful.
I create user personas early in the UX process, usually right after stakeholder interviews. They can and do evolve as we discover more about real users during user testing, and they are helpful in making predictions and forming questions for later stages. As we find new information I update the profiles and document changes.