Picture the scene:
You're walking home from school. You walk alone as you don't live near any friends, so you spend the 30 minute walk daydreaming. Despite walking alone the streets are full of other school pupils, both from your school and the other secondary school in town. Your route also takes you alongside many roads, filled with cars full of people.
During this particular walk home, you are relaying a conversation in your head. Or rather, you are thinking of all the things you should have said. And the reactions to these things you should have said. Before you know it, you are conducting a full on conversation in your head, with you playing the role of both yourself and this other person, answering on their behalf what you want them to say. This carries on the whole way home. You notice people looking at you but ignore it- this conversation game is way too satisfying to think about what's happening around you.
Just as you are about to get home you walk past a window. Still in full on conversation mode you glance in the mirror. Suddenly you notice something, something you wish you hadn't. You were mouthing the words that were being spoken in your head. And pulling the relevant expression (anger in this case). And you realise the whole way home you've been mouthing words to yourself, and pulling a spectrum of facial expressions, from anger to love to hysterical laughter. Without even realising.