Does the interview for a US B1/B2 visa actually influence the decision?

Does the interview actually make any difference to the outcome of an application?

From my experience, the main purpose of an interview is to fill the missing gaps left in your online application and double-check it. As such, it is unlikely you will increase your chances by being nice and charming with the interviewer, but you may fail it if you can't defend what you wrote on your DS-160, as well as the legitimacy of your previous stays.

Common questions asked on an interview should include such things as where you are going, for how long, for what purpose, and you will be expected to answer them.

I noticed that many interviewers have a bad command of the country's local language, though they'll sure as hell not tell you that. If you notice the interviewer has trouble understanding what you're saying, be ready to speak in simple sentences or switch to English entirely, ideally without making them feel like an idiot.

Trivial questions aside, it seems a common tactic among interviewers to challenge one of your statements, throwing the presumption of innocence out of the window and trying to accuse you of lying. They may question your claimed purpose of visit, or conveniently fail to find your last H1B extension authorization, accusing you of severely overstaying your last visit. If that happens, stay calm and patiently defend your point, ideally backing it up with the documents that you brought with you.

how best to prepare?

Bring as much paperwork as you can. Ignore the USCIS guideline that says to bring the bare minimum. If you visited the country in the past, bring all the documents proving your stay was legal. If you were invited to a conference or a job interview, print the entire e-mail exchange or get an invitation letter. Bring your bank statements.

Once you've prepared the paperwork, make sure you can coherently talk about your visit and answer basic questions.

Once everything is done, all you can do is relax, practice mindful breathing, and focus on the enjoyable part of it. Getting a US visa is a demeaning process filled with prejudice, extensive profiling and paranoia, unless you come from a visa waiver country. However, if you've already made the decision to visit the US, you might as well enjoy the ride, no matter how rocky it is, and treat the visa acquisition process as a learning experience, regardless of how well it goes.