In Helaman 7, we learn that Nephi and Lehi had returned from the land north earlier. This was when Nephi was so shocked at the wickedness he found back in Zarahemla, that he went up on his tower to mourn over the Nephites who would not repent and to chastise the people. Several years later, Nephi was probably at home in Zarahemla at the time Samuel spoke there from the walls of the city.
If Samuel had been a convert of Nephi, perhaps Samuel was with him or had come to visit Nephi and Lehi on their way home from teaching in the land northward. He may have been shocked, as Nephi had been, when he discovered how wicked the people had become in the city of Zarahemla. He likely prayed about the situation, and was told something like, “Samuel, I need you to deliver a serious message—and there is bad news. These people must repent. But there is good news—I want you to explain specific signs that will shortly appear so that the people will be able to believe and know that Christ has come into the world.”
Book of Mormon Central, “What does the Book of Mormon Teach About Prophets? (Helaman 13:4),” KnoWhy 284 (March 8, 2017).