I have been a security clearance reference a few times for a neighbor who works at the Pentagon and they wanted to speak to a neighbor. I don’t know what level of clearance she wanted, but I’ve done it at least 4 times (every time she gets promoted I guess).
The interviews have all been over the phone and lasted about 60-90 minutes. Very intense and many wacko questions (re gambling, drugs, affairs, criminal activity, prostitutes, their sexual fetishes, etc). They wanted to know if my neighbor partook in any activity that is blackmail worthy, dishonest or illegal. Some questions just blew my mind.
I kept telling them “I’m just their neighbor, I have no idea”. So be ready for the investigators to ask your neighbors all sorts of nuts stuff.