If Joseph Smith dictated the Book of Mormon on the fly, why would he make it so difficult for himself? One unnecessary complexity when dictating is to include editorial promises, or previews of what will be coming later in the book. We see an example of this in Mosiah 28:9
And they took their journey into the wilderness to go up to preach the word among the Lamanites; and I shall give an account of their proceedings hereafter.
Here Mormon sets an editorial promise to give an account of the mission to the Lamanites but it is not until 18 chapters later that this promise is fulfilled. Why would Joseph do this if he was creating the content on the fly? Wouldn’t he be taking an unnecessary risk? Why would there be over 20 editorial promises in 1 Nephi alone?