If your client hasn't already done so when initially booking the chat, it can often be a good idea to send you client message (using the message function within AirSideChat.com, under "Clients") before the chat takes place. In the message you can ask about their particular interest and maybe their existing level of knowledge on your expertise. With this information, it's far easier to make the chat closer to the client's expectations.
Sometimes you will find that clients are exceptionally talkative and might be looking for an expert simply to listen and bounce their ideas off. Other times you may find that the client is hoping to be on receive the entire time while you do all the talking. These chats will require a little more effort on your behalf but it's actually remarkably easy to talk about something you know intimately. In reality, most chats are like any other normal conversation with someone you might might meet in the pub; relaxed, enjoyable and a learning experience for both parties.