What Do I Do if An Interviewee Doesn't Want to Participate in My Investigation?

It's easy if someone doesn't want to participate in an interview with you. You need to make sure that this is true, but generally it is. When someone doesn't want to talk to me, doesn't want to be interviewed, I just say, "Your employer expects you to participate and are you sure that you want me to report back that you failed to participate in an investigation?"
And it's usually a duty of investigation with any sophisticated employer. With a sophisticated employer, this will be a part of their handbook. It'll say, "If there's an investigation, we expect you to participate." And so, usually it is one of those things where it's a reasonable instruction from an employer to participate and termination from the employment is an option if they fail to participate. Again, there's usually more wrapped up in that and I do not ever threaten anyone's employment. But, generally speaking, if somebody doesn't want to participate, I simply need to remind them that it's a reasonable expectation that their employer has asked them to do.