I think initially we have a problem saying the specific word or sound which then creates the anxiety around said sound or word. I think most of the time in normal conversations, stutterers even aren't thinking problem words that might come up because conversational speech is spontaneous and we're focused on communicating a message rather than words needed to say that message. I think feelings of anxiety around speech sounds only come into play when we have moments were we plan our speech for presentations or interviews or any other already planned speaking event.