When speaking in public, you do not have the same immediate feedback from your audience. The public audience is diverse, you only hear from a few of them, the ones you hear from are not representative, and you don't get their responses in real time. As a result, where the internalized interlocutor in your head should be, instead you have a vacuum. The natural mechanisms for internalizing an audience don't work, and the results can be painful. You may sit down to write an op-ed column for the newspaper, and find that nothing comes out, or what comes out sounds nothing like an op-ed column. You aim, but you shoot wide, and the result doesn't even sound like you. You *feel* that vacuum, and it sucks all kinds of paranoid fantasies into it. That is where stage fright comes from, or freezing up at the idea of contributing to an online forum.
Since the internalized interlocutor is so often made up from only what one thinks about another person, not their real internal experience, it's easy for the imaginative sort to construct one out of people never met. Oscar Wilde always gave the impression that his funny bits were things he found funny, not jokes told for the amusement of the masses -- at least not on a first-order level, only on that higher-order Freudian etc. etc. level.