Explain your current understanding of who does what, maybe by pointing to simple diagram or sketch, such as an interaction diagram(Figure 3.1; see Section 2.3 for details). It may help if you draw the diagram in front of the interviewee, as this shows that your view is not fixed and allows them to intervene. You can then encourage the interviewee to say whether your understanding is right, and if not, in which way it could be improved. After that, you can get them to describe what they know and do, and to point out any difficulties they have experienced using the current system.