Visually signal that you are concerned, understanding, and would like more information. Ask expansive questions to go deeper. I also like to explore if they have given any feedback to the attorney about the area in question. Then key your follow-up question to their answer. If you feel unclear about the client's concerns, interact with them about your intentions.
Anonymity is the mother of trust.
Always offer the client anonymity when you lay the ground rules at the beginning of the interview. In my experience, about half of the clients will invoke this privilege when they reveal negative feelings. When they bring up anonymity, reaffirm your commitment to them verbally. Then, turn off the tape recorder. Show them that you respect this request.