When I do interview coaching with people, they sometimes mention that they feel overwhelmed by the number of things they're supposed to keep in mind during an interview. They have to remember information about the company, try not to say "umm" or "like" too much, keep their answers organized, stop fidgeting with their hands, maintain appropriate eye contact, remember the stories they want to tell, and absorb new information about the position. As if they weren't nervous enough just to do an interview in the first place!
To address this concern, it's important to separate out those things that actually should take up valuable space in your mind during an interview from those that you can work on prior to an interview but which you shouldn't focus on in the moment.
Vocal ticks — things like filler words (um, you know, so...), vocal upturn (which makes your statements sound like questions), and speaking in a monotone — and nervous physical habits like picking your nails, playing with your hair, or swiveling your chair are all great things to practice overcoming to help you appear more confident in an interview. But they aren't important enough that they should be your primary focus during an interview.
For example, when I taught a class on interviewing, my students had to do a final videotaped mock interview, and I made them sit in swivel chairs so they could see how distracting a nervous habit like swiveling back and forth in your chair can be to the interviewer. One student became fixated on this and was so proud of himself when he finished his interview without swiveling his chair. That was great, but his interview itself was not! He had made keeping his body still his top priority to the detriment of having polished, enthusiastic interview responses.
Part of the reason I advocate practicing several times before an interview is so that you have time to work on these distracting habits until you naturally look and sound more confident. It's like driving a car: When you're first learning how, you have to think about where to keep your hands, when to press the pedals, and how to scan the road ahead of you, and you wouldn't want to try to have a serious conversation with someone at the same time because focusing on the driving and the conversation at the same time would be impossible. But once you've practiced enough and driving has become natural to you, you are able to have a conversation with a passenger without putting your life in danger.
During an interview, your main focus should be on effective, enthusiastic communication of your strengths and abilities. Whatever progress you've made at that point in conquering your nervous habits, let it be enough, and keep your attention on how and what you're communicating.