Honestly, theres nothing wrong with um's, even interviewers say um's, don't worry about that! Do whatever you feel helps you! If saying um helps say um's :). Also don't be afraid to take pauses in between and don't be afraid to repeat sentences if you feel a block. The interviewer will probably just think your nervous, it happens to the best of us. He/she will probably think its because of the nerves :) . a stutter will hardly be the first thing that comes to mind, it usually never is. Rooting for you!