It is interesting but let me offer another scenario i just felt.  It was very light and easy to get over but right on que!  I am on the phone and as I was talking I saw someone come out of the office. The moment I saw them, my stutter quickly began.  Was easy to overcome, it was a little blip but fascinating none the less because it's something I know many others go through.  I remember in vc, the moment someone else entered, the person stuttered or stuttered more, really like on que from someone else entering the chat like a trigger.  So it makes me wonder how fast any conditioned fear aspect could appear or if it was a processing issue maybe not even related to fear but a disturbance in processing power of the brain?  I was more excited to see her from the office and wanted to say hi.  I also remember this kind of thing happening when I think ahead too much.  When I think of the multiple things I have to portray, it can trigger the stutter, when I feel non-challant one thing at a time, it is usually not triggered unless something else interferes in my state.   I know sometimes doubt triggers it but when I saw the person..it felt more like a processing issue.  I wasn't afraid of her, I wanted to actually say something.  But something about being mid convo, seeing them, processing what I wanted to say to them all at the speed of light felt like it triggered a slight stutter.   I think when others walk in on others, it may be a doubt/fear aspect too...maybe this is learned but...it is so obvious in others that I wonder if it's more a processing issue than learned because I wonder if this happens even when people who enter, we are fully comfortable with who say.. know we stutter and so on.