They'll start explaining rather than denying. Emphasize that people are more believable when they put their name behind what they say. A robust public debate. It doesn't hurt to say you need the person's help. When the door is slammed in your face, try again a day or two later.
People change their minds.
Take the subject through his or her story chronologically. You will understand the tale better, and you will spot gaps in the timetable and logic. It sheds light on credibility, extracts more detail and is a door opener to other sources. Also, ask people why they do what they do, rather than just asking what they do. When people reach the important part of a story, slow them down so you can get it in technicolor.