Yes, you could be right the stress or anxiety evokes the conditioned response, which in this case is tightening the lips or other speech muscles needed for speech production. Anyway, what I was trying to explain is, if you - right now - do tense the muscles needed to pronounce the word or letter, then the physical tension itself can never lead to a prolongation or halting of the speech movements. Try it with walking now, stand up now and then walk a couple of steps (1) while tensing your leg muscles as much as possible. See? The movement of the legs (while you were walking) didn't halt, in the same way that the speech muscles can't halt from tensing