The scriptures record Jonah's sermon to the people of Nineveh this way:
Jonah 3:4-5 (ESV) 4 Jonah began to go into the city, going a day's journey. And he called out, “Yet forty days, and Nineveh shall be overthrown!” 5 And the people of Nineveh believed God. They called for a fast and put on sackcloth, from the greatest of them to the least of them.
Should we assume this is really all that was said or is it safe to assume that maybe the people had some follow up questions and Jonah had some follow up answers?
It's interesting that Jonah doesn't even mention God, yet the people know to repent to God. Perhaps they were already acquainted with יהוה (YHWH) and the Israelites similar to the acquaintance of Rahab and the inhabitants of Jericho so that all they needed was an Israelite to show up and forcefully proclaim calamity.
Joshua 2:9-10 and said to the men, “I know that the Lord has given you the land, and that the fear of you has fallen upon us, and that all the inhabitants of the land melt away before you. 10 For we have heard how the Lord dried up the water of the Red Sea before you when you came out of Egypt, and what you did to the two kings of the Amorites who were beyond the Jordan, to Sihon and Og, whom you devoted to destruction.