Here is an example, from Helaman 2:
12 And more of this Gadianton shall be spoken hereafter. And thus ended the forty and second year of the reign of the judges over the people of Nephi.
13 And behold, in the end of this book ye shall see that this Gadianton did prove the overthrow, yea, almost the entire destruction of the people of Nephi.
14 Behold I do not mean the end of the book of Helaman, but I mean the end of the book of Nephi, from which I have taken all the account which I have written.
These are called editorial promises, and there are many of them throughout the book. I'll highlight two editorial promises in this post.
Mormon's editorial promise about Gadianton
Notice the wording in verse 13: "this Gadianton did prove the overthrow, yea, almost the entire destruction of the people of Nephi."
This is a perfect opportunity for Joseph Smith to accidentally insert a big plot hole into the book. If he had merely left out the word "almost", we'd have to explain how Mormon, who is killed 96 chapters later (prior to the entire destruction of the Nephites), knew the full extent of the damage Gadianton would do.
By including the word "almost" in the sentence, Mormon reflects an accurate understanding of the effect of Gadianton up to the time of writing, leaving it to Moroni to describe the brutal end of the Nephites. It's a subtle detail, but a very important one, and the Book of Mormon gets it right.
I believe Mormon had an outline. He reviewed all the records than decided what to include in his record.
In some cases, he included the record word-for-word, such as Alma-2's instructions to his sons found in Alma 36-42 or Moroni's epistle to Pahoran in Alma 60.
Some records he merely summarized, such as the period of wickedness leading up to the massive destruction at the time of Christ's death.
Other events he left out completely. He gives us a long list of what he is leaving out in Helaman 3:
14 But behold, a hundredth part of the proceedings of this people, yea, the account of the Lamanites and of the Nephites, and their wars, and contentions, and dissensions, and their preaching, and their prophecies, and their shipping and their building of ships, and their building of temples, and of synagogues and their sanctuaries, and their righteousness, and their wickedness, and their murders, and their robbings, and their plundering, and all manner of abominations and whoredoms, cannot be contained in this work.
15 But behold, there are many books and many records of every kind, and they have been kept chiefly by the Nephites.
A detail like this one would be really hard to keep track of by a person who is attempting to dictate the book in a single draft without notes.
An editorial promise made by Mormon and fulfilled by Moroni
Mormon's outline seems to include a list of what he had promised to include. When he died in battle prior to completing the work, his son Moroni took over.
I believe Moroni was able to complete the record by reviewing the outline and list his father had kept. This brings us to the second editorial promise I'd like to discuss in this post--an example of Moroni fulfilling an editorial promise made by his father. (Many thanks to Stisa for pointing this one out to me.)
In 3 Nephi 18:36-37, we read about Jesus Christ touching the disciples to give them the power to give the Holy Ghost:
36 And it came to pass that when Jesus had made an end of these sayings, he touched with his hand the disciples whom he had chosen, one by one, even until he had touched them all, and spake unto them as he touched them.
37 And the multitude heard not the words which he spake, therefore they did not bear record; but the disciples bare record that he gave them power to give the Holy Ghost. And I will show unto you hereafter that this record is true.
The bold words above explain what happened. The italics tell us that the multitude did not hear what Christ spoke to them. The underlined words contain the editorial promise.
In Moroni 2 (39 chapters later), we read the fulfillment of this promise:
1 The words of Christ, which he spake unto his disciples, the twelve whom he had chosen, as he laid his hands upon them—
2 And he called them by name, saying: Ye shall call on the Father in my name, in mighty prayer; and after ye have done this ye shall have power that to him upon whom ye shall lay your hands, ye shall give the Holy Ghost; and in my name shall ye give it, for thus do mine apostles.
3 Now Christ spake these words unto them at the time of his first appearing; and the multitude heard it not, but the disciples heard it; and on as many as they laid their hands, fell the Holy Ghost.
In my opinion this example is powerful evidence that the author had notes and time. In my opinion, a good theory for how we got the Book of Mormon text will account for this type of detail. The theory espoused by believers accounts for this in a pretty simple way--Mormon was the author and he had a lot of notes and a lot of time. Naturalistic theories fall short.