Be honest. You will be placed under oath. You are under the obligation to tell the truth just as if you were in front of a judge and jury in court. If you intentionally lie, you can get in trouble for perjury – lying under oath. Sometimes, the only truthful answer is “I don’t know.”
Answer only the question that is asked. You need to stay within the scope of the question. You do not want to get conversational.
Take breaks. You are not a hostage. If you get mentally tired, need to go to the restroom, or need to talk to your attorney, take a break. As a rule of thumb, I never go longer than an hour without taking a break.