Never interview a customer, just have a chat - Me
Client or customer interviews can be the hardest to master. You can go in too hard and put people off or you can go into soft and get no-where. Interviews play off your own social skills, really dive into this and the way you are talking to people; be inquisitive, spot their passions, unlock a commonality. Note - Interview is a strong word, try not use this at all when setting up or discussing with customers
There are multiple types of scenarios when you might interview a client or customer (these might even only be potential clients at this point) these can vary from initial chats just on an offer statement to full on requirements gathering at a very low level. Try to tailor each interview to meet the requirement at the time. Below is a high level summary of what you might do depending on the scenario.