If Joseph Smith was dictating the Book of Mormon on the fly we would expect some inconsistencies in the storyline or in the finer details of the places he was describing.
Amazingly, Joseph mentions 26 journeys to and from Jerusalem in 1 Nephi and is consistent with his usages of “up” and “down” in each one. There are no signs in the manuscript that he corrected himself. Evidence Central lists the 26 consistent instances of going “up” to Jerusalem, and going “down” when moving away:
What are the odds that Joseph wouldn’t slip up once during dictation?
See: