In marriage visa interviews, the government tries to have them conducted in a uniform manner. With that said, most experienced examiners naturally use their own experience and knowledge to guide the interview process. Immigration officers performing these interviews have considerable training and specific requirements. They know what needs to be seen on certain documents and certain information needs to be given during the interview. Generally, the USCIS does a very good job of determining whether a marriage is bona fide.
There is some discretion given to the officers to collect all of the facts and then making their decisions. Overall, they are well trained in doing it in a consistent manner. But if an officer believes a case is deniable, they must usually seek confirmation by presenting the facts to a superior examiner.