Interview time is not social time, and a mistake that younger narrative reporters frequently make is to be too nice and too obliging and to act too much like the guest and not enough like the reporter.
I tell my sources up front, don’t tell me what you don’t want me to know. Or tell me now if there is an area that you want to wall off, and I’ll see if I can continue working with you.
If they start feeling more comfortable, it’s a problem. You’re liable to be putting yourself at risk of having contaminated access if you go too far. It is possible to be a poised professional friend and acknowledge that. If you go over a certain line, you may be in ethical trouble. I don’t say anything goes. I say that there is a matter of honor and decency, and your readers will be your judges.