I am not a Visa officer, but I have been in front of one for about six times, put together visa applications from scratch for multiple people, filed them through to completion, replied to RFE’s, Filed GC applications etc. Hope this answer serves as a decent placeholder till an actual officer at an embassy answers.

In most cases, the Visa officer simply does his/her job. they have clear procedures to follow when evaluating an applicant and the case presented lets him make a straightforward decision regarding approval or denial. For instance,

• A guy who was attending an interview for a H1B visa while I was waiting in line. From what I observed, this person was apparently employed through a second level contractor, who was in turn hired by another contractor to work on a project for IBM. He was not able to even give clear answers to who had actually sponsored his visa and how much he was being paid. After persisting with a few more questions, his visa was rejected.
• The visa of a friend who answered that he planned to settle down in the US after going there for his masters degree on a non-immigrant F1 Visa was also rejected.
• On the other hand, almost 40 odd people I know, who were in full compliance with any rules/restrictions for the specific visa they were applying to, were able to give clear, confident and unambiguous answers to basic questions centered around the rules/requirements were approved without much hardship.

Apart from these clear cut cases which allow for an easy decision, visa officers also usually encounter a small but significant number of more complex cases which require them to use their discretion and judgement in making a decision with the facts presented. In these instances, the following skills/factors usually play an important role.

Training

Visa officers are pretty well trained in asking the right questions pertaining to issuing a specific visa. They have a thorough understanding of rules, requirements, procedures, documentation and visa policies. Their training usually makes them pretty good at cutting to the chase and drilling down with additional questions regarding a particular area of concern they have about an applicant.

Experience

The clever ambiguous answer you come up with to avoid calling attention to a shady omission you made in the supporting documents has probably been heard by your officer hundreds of times. The fourth dubious application filled in with the same errors by the same ‘visa consultant’ is probably not that hard to spot and reject.

Intuition

Working in the profession long enough creates a pretty accurate sense of intuition about every applicant. Just like an experienced doctor or lawyer, they are able to pick up almost imperceptible red flags such as say your body language, hesitation while answering etc to ask additional probing questions which check if everything is actually right about an application

Erring on the side of caution

It is usually more important for an officer to keep the wrong people out, rather than having to worry too hard about a couple of legitimate applicants who were forced to reapply or provide additional documentation. This situation often makes them deny a Visa even when they are left only with a small amount of hesitation after the interview. It is simply not their mandate to make the perfect decision on every application, especially when the people they reject often have the provision to apply again.

Environment

The behavior of an officer may be completely different based on the country his embassy is in. for instance, a US embassy in India or Mexico usually see a lot more questionable applications and officers here maybe a lot more skeptical than an embassy in say Japan or Australia. The approval rates of embassies of a country vary greatly based on the country they are located in.

If you are following this question to because you are applying for a visa and are interested in knowing how to improve your chances:

Having played this immigration game long enough, if there is one thing I have learnt that would be the understanding that >95% of advice found online about the Visa process/interview is complete BS. It is usually created by people based on second hand opinions, intended to cause undue panic among other applicants and usually far detached from reality. The next time you read about how

• Workplace photos are absolutely necessary for getting your H1B stamped
• You cannot set foot outside the US because you are on an EAD
• Make sure you are dressed in a suit when you go for your visa interview

Please disregard them and kindly follow only the rules/advice mentioned on officially recognized sources such as the USCIS website. The best way to be prepared for a visa interview is to fully understand the eligibility criteria for the specific category of visa you are applying to, having the required supporting documents and being prepared with clear, concise answers to questions centered around establishing your eligibility.