Jonah started into the city, walking one day, and he cried out, “Just forty days more and Nineveh will be overthrown!” Jonah 3:4
In his assignment to go to Nineveh, given a second time, God had told Jonah that he was to, "declare against it the proclamation that I am commanding you.” As promised, God gave Jonah the proclamation he was to proclaim, “Just forty days more and Nineveh will be overthrown!” This is what he preached on the streets of Nineveh. God had given Jonah a word to deliver to a group of people and he delivered it.
Jonah, "just did it". He didn't write it, he didn't look for key people, he didn't whisper, and he didn't try to be contextual in assimilating to the culture and finding a way to sugar coat this hard word. No, he just did and said what God told him to do and say. Was Jonah smiling or frowning, stern or genteel? We don't know.
Prophetic preaching is when God gives you a word concerning the future of a people and you proclaim it to them. The preacher, proclaimer, or messenger has to be careful not to add his or her words to God's word. This is just as important in prophetic ministry to individuals, couples, families, or small groups. The prophetic minister may often need to say, "I don't know what it means". Your interpretation of the word, if it needs interpreting, is not part of the word from God; and that needs to be made very clear to avoid confusion. So, a true word from God can be misinterpreted.
Prophetic ministers, proclaimers, messengers, or writers can pray to God for the interpretation. God may answer and give the interpretation or he may not and the person who received the word may have to submit his prophesy to the community of other prophets. In fact, we are admonished to weigh each others prophecies in the New Testament. In Christianity we have common prophecy in community where other prophetic people help to administer prophetic words. See 1 Corinthians chapters 12-16.
What about the good news? How does Jonah apply to His-story, or salvation history? Jonah's message was a wake up call. A wake up call is sent by a loving God. God cared enough to send a message to a sinful people, telling them that time was up for them, in forty days.
If God cared enough to send this message through a person, then perhaps the message is true? The sinful people are given a chance to repent before they are destroyed. Destruction is not good news, but the warning is merciful.
God has suddenly, "brought the bottom up", for Nineveh. Their sin has finally brought severe consequences. God perhaps engineers or allows circumstances for people that are rude awakenings. We need to help people turn to God for salvation and not save them ourselves. The message from God is designed for the recipient from God and the messenger should be clear that they do not write the message, but only deliver it.