Put your focus on what you want to say and not how you're going to say it. You can't control if it's going to be easy or tough in terms of speech, so just keep your mind on your answers and not on your fluency since that's what you have control over. Also if you have your answers down, that'll be one less thing to compete with over brain power if you do start stuttering. You could also tell them that you stutter at the start of the interview if it feels right. If you tell them and you only stutter a little bit, no problem. If you tell them and you're stuttering a lot, well now they already knew to expect that.