What people who don’t write don’t understand is that they think you make up the line consciously — but you don’t. It proceeds from your unconscious. So it’s the same surprise to you when it emerges as it is to the audience when the comic says it. I don’t think of the joke and then say it. I say it and then realize what I’ve said. And I laugh at it, because I’m hearing it for the first time myself.
I find this to ring true for all types of writing, not just comedy. The article or story writes itself in my head; it’s up to me to listen to it & transcribe. That’s the only way I’ve ever been able to describe being a writer, and why it’s so hard to write on command. It’s also why – at least for me – exercise, routine & being with myself – is the best way for me to be able to “hear” the words that are being written.