The book "Self Therapy for the Stutterer" explains the fear of speaking so a fluent person can understand what a person who stutters goes through.
A person who has nothing wrong with his legs "could walk easily without trouble or quite 'fluently' along a long plank, twelve inches wide, when it is placed on the ground.
But if that same plank is placed on a wall high above the ground, then, if asked to walk along the plank, he would probably develop a fear of falling. As a result, it would be difficult for him to walk along the plank in a normal way. In fact he would probably put on a poor exhibition of walking normally or 'fluently.'
This example is somewhat parallel to the problem of a stutterer. Nearly all stutterers have the physical equipment to talk properly, but in most cases the fear of stuttering causes them to try to force trouble-free speech. And since it is difficult to force the articulative mechanism of speech, he does not speak fluently."