I experience the same thing. I don't stutter when talking to pets or myself, but as soon as I speak to a human, I get locked up. It's definitely caused by fear/anxiety. I've noticed that when speaking to others, I have the tendency to try to talk too fast. So it helps when I make a conscious effort to slow down my breathing and rate at which I talk. I mostly get stuck now on words that begin with B or P, and especially if they're double consonant words, like Br- or Bl-.