First of all, thank you for the feedback everyone has posted. I wanted to make sure I shared my experience in order to be able to contribute as well.
I was on a work visa for a few years now and then met a girl who I got to know for over two years. We got married in Jan of this year. We filed for an adjustment of status on Jan 31 and received our interview date for the beginning of April. After a few weeks of receiving our interview date, the USCIS requested more documentation to prove our marriage (bona fides) and asked us to bring this proof to the interview. This last letter made me nervous because I thought maybe they would give us a hard time if they are going out of their way to ask for documentation.
We prepared meticulously. We had bank statements, gym memberships, car loan documentation and other such admin docs to prove our finances were co-mingled. We got affidavits from our landlord and friends who knew about our marriage. We even printed correspondence of when we first met (emails) and itineraries for the past two years. Then we printed over 200 photos and labeled them with all of the trips we have taken together as a couple.
The interview did not last for more than 15 minutes. This is something that I would request other posters to write down. I couldn't find, in your experience how long the interview should last.
Here is my advice:
1) Prepare your file and be confident. If you are honest and truthful, you have nothing to fear
2) Use an attorney if possible, in order to help prepare your file. We did not bring them to our interview because it would be too expensive but they assisted us in filing all of the right work. It is worth the investment.
3) I don't find enough people have said this: Present yourself well. Dress neatly and professionally (I recommend a suit or business casual at the very least). Be well groomed. Smell nice. This makes a good impression. Dress well! You will feel better during the interview too :)
4) Speak clearly and concisely. Be respectful in your responses to the officer. Feel free to use "sir" or "ma'am/madam" as a sign of respect.
5) Ensure that you know exactly where all of your documentation is maintained. When they ask for a document, have it at your fingertips. This requires, good labeling
6) Smile and maintain a polite demeanor during the interview. If you don't understand the question, clarify it. These IOs are not bad people. They are just doing their job. Don't take anything personally if they ask you a tough question. Relax and answer honestly and to the best of your ability. If you don't remember, say that. Don't guess.
7) They used a video camera for our interview and said it would be recorded. Ignore the camera and forget it's there. One way to do this is to focus on the IO.
8) Practice meditation and breath work to stay calm because I can see how it can be intimidating. What I did, to maintain a positive state, is that I spend some time looking at videos that made me laugh and was joking with my wife (not loud or inappropriately) in the waiting room. The morning of the interview, I watched comedies on YouTube.
The IO was very nice and we were nervous after having read some horror stories. He spent some time (very little) going through my folders but then you could tell that he made up his mind and was satisfied by the level of detail we had provided. He asked us simply our DOB and when we were married, who attended, and then re-confirmed the question on the forms we submitted (criminal record, history with weapons etc.). It seems that they have enough experience to gauge who is lying and who is telling the truth.
So be prepared, then relax, dress well, look good and be confident.
Thank you all who have provided their advice. God Bless!