According to John Tvedtnes, if we presume that Mormon had access to ancient records, it seems likely that he would sometimes selectively rewrite the stories in his possession, while, at other times, he would paraphrase or abridge them. In some cases, he might wish to quote extracts from the texts. In all but complete rewrites, evidence for Mormon's hand might be reflected in the switch between first- and third-person accounts.
An example of this phenomenon can be found in Alma 56:52:
And it came to pass that the Lamanites took courage, and began to pursue them; and thus were the Lamanites pursuing them with great vigor when Helaman came upon their rear with his two thousand, and began to slay them exceedingly, insomuch that the whole army of the Lamanites halted and turned upon Helaman.
This passage mentions Helaman by name and speaks of him in third person, despite the fact that it is in the middle of a letter (Alma 56-58) written by Helaman in which all other references to him are in the first person. It is likely that Mormon, when including the letter in his account, simply slipped into the role of historian and, in retrospection, employed third person this one time. [John A. Tvedtnes, "Mormon As an Abridger of Ancient Records," in The Most Correct Book, p. 13]