When I took my interview with my wife and kids, I was asked if my income met the minimum requirement (125% of the povelty line) and how genuine my marriage was.
For my case, I showed the officer my last 3-yr tax returns, my employer's letter, my newest paystub, my kids born in US, co-ownership of vehicles, bank accounts, investment accounts, life-insurance beneficiary, co-signed rent contract, etc.
If your materials for your I485 are true and you tell the truth, the interview should be very easy. There are no trick questions. Don't panic.
Your passport may get stamped if your background checking is finished.