I will often stutter on closed questions, the answers to which don't give you any linguistic freedom, like being asked my name, or age etc. For open questions with freedom to substitute words 'in an emergency' I don't tend to stutter and even most of the time answer exactly as I originally thought, *without* any word sub. I think it's stress related - subconsciously knowing that the answer to a question is totally flexible reduces stress and seems to almost eliminate the stutter.