You can always ask them what they would prefer you do but this could be a little awkward depending on the person. I think the best thing to do is just to have patience, to listen without trying to finish their sentences. What also helps is to speak in a calm slow voice to them. It can help that person to speak more fluently (this is a thing that helps me because I can sort of mimic the other person's voice and way of talking and it sets a non-stressful tone to the conversation)