User support differs from many other roles in that a standard interview actually can be a very good way to evaluate a candidate: it gives you a chance to demonstrate clarity and succinct answers to questions you initially have no control over. Remember that your interviewer (should) be looking for someone who can quickly & clearly answer a question - so always be brief rather than expansive [which is not always good advice for other roles]. I recommend mentioning the importance of gently (but firmly) controlling the direction of a support conversation to identify the core issue & efficiently identifying the best response - resolve/escalate/apologise/whatever. Then if possible demonstrate that skill within the interview - so if subsequently asked a vague 'open' question, politely ask 'closed' questions to narrow down the real substance behind that question, and then answer that decisively. Good luck!