It really depends on the level of security clearance, I think. I had a secret clearance years ago, and never heard from friends or acquaintances that anyone had asked about me.
I agree with this. I got a security clearance with the Canadian government, and the only way they could have interviewed my friends was to find them across EU and the world. They relied on police check, including federal, and fingerprints.
The investigation for S is pretty tame. But for TS they do a lot of interviews. I had a long conversation with an OPM investigator about one of my friends' past drug use (he got the clearance).
