Few thing very important for the interview
1) Always tell the TRUTH, never lie to the officer
2) Listen to the officer carefully and answer properly and politely to each question
2) Be confident in your replies
4) Do not provide extra information other than the one asked
5) Keep eye contact.
6) Keep ur response short and to the point.
7) Look at the officer in the eye when speaking to them.
8) Always be polite,no matter whatever they are asking.
9) Dress appropriately go for business casual use minimal jewelry.
10) Be mentally ready for the long wait on the day of interview
11) Try to reach the consulate a bit earlier than scheduled time
12) Whole interview is to check whether the relationship is genuine or not so if your relation is genuine you don't need to panic