The single most important thing you can do to sound intelligent and well-prepared for an interview is practice. You’ve heard this before, but what does “practice” actually mean?
Preparing for questions is an obvious tip, but most people make the mistake of preparing their answers only on paper or in their head. Many people do not actually speak their answers aloud, with their voice. This is important. The difference between thinking about something and speaking aloud about it is vast. When you speak, you often hear things differently. It might take you a few tries to actually organize what you want to say. If you do this for the first time in an interview, you will look unprepared. If you practice explaining aloud beforehand, you will have time to organize your thoughts, choose the correct words, and ignore the unimportant filler words.
As an added bonus, practicing aloud can actually help improve your memory (see this study). If you want to remember your answers, speaking them aloud will be beneficial.