You have your BOLI complaint in hand, and here we go:
1. Read the complaint very carefully. Typically, the first page will tell you who has filed the complaint, and how to respond. It will also tell you when your response is due (typically in 14 days from the date of their lette)r. The second page will give you the actual allegations, "Unlawful employment discrimination..." It is urgent that you read and commit to memory the basics of what the employee/ex-employee said. This will be helpful as you begin your interviews.
2, Determine where you will find answers.
2. Start a list of questions you want to ask relative to the written allegation(s). Make them open-ended, don't lock yourself into "yes" or "no" answers unless it's necessary. Review your questions to be sure they are not leading and that they will give you what you need.
3. Start with the Complainant. Meet in as comfortable and quiet an environment as possible. Before meeting, clear your mind of other things. Slough off any assumptions. Walk in with only one goal in mind: understanding what the complainant's view is. Drill (gently) with clarifying questions until you are convinced you know all there is to know from his/her perspective.
You get the idea. Just don't let statements drop without follow-up questions.
Let the Complainant know what you plan to do, what your role in all this is.