They may not know a single detail about the interest. Get them started with a little preliminary research and developing a few key questions. It will get the process going and give them a sense of confidence. It will help them feel better about making phone calls to ask for Informational Interviews. See the Making it Happen chart on pages 6 and 7.
Students actually develop something of an expertise when they pursue an interest.
They learn the language of that interest area, they read about it and they delve into some aspect of it. It all starts with a few questions that they themselves develop. Think about having each student keep a language log of all the new words they pick up as they pursue their interest. That's vocabulary development in the real sense of the word. There are many ways to pursue an interest.