For me too I am in such a habit of this that I catch myself doing so walking down the street and on the bus. Hopefully I have never moved my lips in public. My favourite place to self-talk is in the shower.

I do not create scenarios so much as 'rehearse' my reactions to those that already exist.

Most often it is rehearsing how I might stand up for myself or explain to someone my case or situation. An good example is if I have something I need to say to my psychiatrist or practise and prepare for a conversation I need to have on the phone. I will prepare for every possible reaction and outcome I can think of making contingency plans all along the way. But, once faced with the situation in real life what I have/need to say rarely goes how planned.