If you don't know the answer to a question, say so. Don't guess at the question either. If you didn't understand or fully hear the question, let the officer know. Officers become irritated -- or worse, suspicious -- when an applicant changes an answer after being caught in a contradiction and then blames it on misunderstanding.
Know Your Language Limitations.
If English isn't your first language, decide realistically if you will need an interpreter. If you do bring an interpreter, let the interpreter finish translating each question before answering, and stick to your best language instead of jumping back and forth between it and English.
Choose A Skilled Interpreter.
Know that some officers will let the person petitioning you do the translating, but others will insist that a person besides the petitioner or the attorney do it. That person can be a family member, a friend of the family, or a professional, but must be over 18 and with legal status. Above all, make sure your interpreter has experience and confidence. This job takes more than just speaking both languages fluently. It is difficult and demanding, and if the interpreter makes mistakes it could destroy the interview.
Answer Only What's Asked.
Officers have a lot of questions and don't like being slowed down by answers that are off the point. If you are asked when you came to the country, the answer is a date. Don't start talking about why you came. If you are asked if you ever worked in the U.S., say yes or no. Don't explain how you got the job. If the officer wants to know, those questions will come. The only exception to this rule might be when talking about how you met your spouse (if this is a marriage-based case). In that case, the officer is looking for a story and you should give one.
There may be exceptions, but most officers are not known for their sense of humor. Just as at airport security, save jokes for another time.