Actually when my conditional status was converted to permanent status, it was done without an interview.
The only "interview" I had - from the initial application in England to receiving my permanent status - was at the counter at the US Embassy after the doctor examination (all applicants have chest scans for TB and an AIDs test to make sure you're not emigrating for medical reasons). I handed over my supporting documentation to prove my sponsor (wife) could afford to support me in the event I couldn't get a job and that was it. The interview consisted of a few yes/no answers regarding the financial documents and tax returns.
When it came time to remove the conditional status from my residency, the documentation I used to support the fact I was in a real marriage was sufficient to bypass the interview process. It's hard to say a person isn't in a real marriage when they have joint bank accounts, credit card accounts, loans, mortgages, utility bills, tax returns and affidavits from friends of the family. That doesn't mean people aren't in real marriages if they don't have all those things, but when you do it's better proof than guessing what colour your wife's toothbrush is and what her favourite fast food might be.