Interaction designers on an XP team choose overall metaphors for the system, write stories, and evaluate usage of the deployed system to find opportunities for new stories. Addressing the concerns of eventual users is a priority for the team. The tools of interaction design, such as personas and goals, help the team analyze and make sense of the world of the user, although they are no substitute for conversation with real people.
Much advice for interaction designers is based on a phasist model of development: first interaction designers figure out what the system is supposed to do, and then programmers go make it do that. Phases reduce feedback and restrict the flow of value. Mutual benefit is possible between interaction design and the rest of an XP team without separating development into phases.
On an XP team, interaction designers work with customers, helping to write and clarify stories. Interaction designers can use all their usual tools during this process. They also analyze actual usage of the system to decide what the system needs to do next. Interaction designers specify a little bit up front and continue to refine the user interface throughout the life of the project.