Most of the IOs are nice guys. Don't worry.

They will try to question your relationship. If you can provide information regarding what they ask, there is no problem.
Follow the instruction to prepare documents. Yoou can bring more with you as you think. However, be sure to give IO the documents they ask, one by one.

Try to recall with your spouse before going to interview about: how to know each other and the dating process to marraige, kids, photos, whatever you can prove your true marriage is helpful. They may ask the same question in the separate interview session.

They are nice guys. Their work is to collect all information they need before they make decision.

Don't worry too much. Business dress. Polite smiles can help.

Good luck,