You can tell them that you need to ask them a series of questions that are going to be very personal and possibly uncomfortable but they need to be asked, that way the person will be prepared for the types of questions they are going to be asked. Then be direct, non judgmental, to the point, for every answer they give you say 'ok', don't be patronizing, you are just there to get the facts from this person. If there is something that they feel too uncomfortable about tell them that we can come back to that in a minute, tell them there is no rush. Professional and sensitive.