As a note, I declared my stutter beforehand as "other disabilities" on the pre-interview form. I think it helped because it took the weight off my shoulders, and also prevented them from assuming it was due to nerves or anything. So consider doing the same; either on a form if you're given one or emailing ahead. You could also ask for reasonable adjustments, such as a slightly longer slot, having water available for you (stuttering certainly drys my throat out), or anything else.