You place your protagonist in a frightening situation. Does s/he:
d. forge ahead, heart quaking, knees shaking, etc.
e. turn tail and run
If you answered (c), ask yourself why.
Think about it. Ever stutter when you're afraid? Know anyone who does?
A true stutterer will stumble over many words, not just the first one in a sentence. There's debate over how much is psychological and how much is physiological, but everyone agrees it's a verbal handicap.
I'm not saying characters can't have weaknesses, but if you're going to deprive a character of a clear and strong voice, make sure there's a reason. Give us a bit of backstory, some context that explains why this person stammers (a traumatic event, a parent who insisted on perfect diction at all times, etc.). Be creative, don't settle for a tiresome cliche.
Better yet, show us your protagonist's unique reaction to a fearful situation. Find the words to make us feel their anxiety. Your writing will shine and your readers will thank you for delicious thrills.