One of the biggest reasons candidates fail onsite interviews is because of a behavior referred to as "making assumptions". I am very aware of this, as early in my career it was one of my main failings as a candidate. I spent a lot of time learning to *listen* and ask questions rather than just jumping in with what I thought was certainly "the right answer". These days in my career, if I am the candidate interviewing, I always wait for the interviewer to finish the question before I jump in; I ask clarifying questions throughout the dialogue, and I tend to ask "is that what you were looking for?" when I answer a question to make sure the interviewer isn’t looking for more from me.
When I meet with a candidate first thing before they start their loop, the two pieces of advice I give them are to listen to any hints or feedback their interviewer is giving throughout the conversation, and to talk through their thought process. Often if a candidate is going down the "wrong path", the interviewer will ask them a question that is geared towards helping them get back on the right path, or a clue as to what the interviewer is actually looking for. "Making assumptions" is diving into a question either without waiting for the interviewer to finish setting up the question/scenario or just jumping into the question without thinking through what all the possible answers could be. It is often driven by excitement, or thinking that you know exactly what the question is about, and could be apprehension about interviewing in general. The impressions that can be drawn from consistently not listening are lack of interest, arrogance, poor collaborative skills, lack of communication abilities. In some large companies, those qualities may be less of an issue than in a smaller organization. But given that almost every candidate I speak to is looking for a highly collaborative environment where they can be mentored, handling an interview by not listening and taking hints/feedback gives the opposite impression.
I know it is frustrating when a lot of companies are unable or unwilling to give you specific feedback about your interview performance; this is why it is often a great opportunity to work with an agency – recruiters will often give their feedback to the agency to help coach candidates. Most candidates want to know "why" companies are so hesitant about giving direct feedback, and the truth is that behavior, communication skills, and personality are all highly subjective. On top of that, there is a chance that a candidate will decide to take legal action against a company if they are given feedback they are not happy with. Recruiting is part of Human Resources, and the single most important goal of HR is to protect the company legally as it relates to human beings.
I recommend finding some mock interview scenarios nearby. If you are in school still, check with your career center. If you are working, talk to friends/colleagues and set mock interviews up; you can do it a library, in a coffee shop, even in someone’s home. Most recruiters or people managers are happy to give an hour of their time to help someone they know improve their communication skills. Set up scenarios and then take video that you can refer to later (there is absolutely no better way to find out what you are like in an interview than seeing it for yourself). Interviewing, like anything else, is a skill. Practice can help you hone your style and become more comfortable with the process.