All prophets experience prophetic visions while asleep. During prophecy they lose control of their bodies, and that opens up their minds to understand their visions.

There are many different levels of prophets. Just like one wise man might be wiser than another, so one prophet might be greater than another. All of them have their prophetic visions either at night in a dream or during the day after having fallen into a deep sleep, as it is stated: “I will reveal Myself to him in a vision; in a dream I will speak to him” (Numbers 12:6). When all of them prophesy their limbs shake, their physical strength is weakened, and their thought processes are interrupted, leaving their minds free to understand what they see. (Rambam, Mishneh Torah, Hilkhot Yesodei HaTorah 7:2)

Prophecy is not communicated through direct speech, but through allegory and imagery. Prophecy is a vision the prophet sees, not speech that he hears. Some of the prophecies in the Bible are accounts of the interpretation of their visions, and others include a description of the vision as well as its interpretation.

The message the prophet receives through prophetic vision is given in the form of an allegory. The interpretation of the allegory is immediately clear to him and he knows what it means. Examples of this are the patriarch Jacob’s vision of the ladder with angels ascending and descending upon it (Genesis 28:12) which alluded to the kingdoms throughout history that would enslave his descendants, the animals Ezekiel saw in his vision (Ezekiel 1), the boiling pot and almond branch that Jeremiah saw (Jeremiah 1:11-13), and the ephah measure Zekhariah saw (Zekhariah 5:6). The same is true for the rest of the prophets. Some of the prophets, like these, stated both the allegory and its interpretation, some only the interpretation, and some, like a portion of Ezekiel’s and Zekhariah’s prophetic messages, only the allegory without stating the meaning. But all of them prophesied through allegory and imagery. (Rambam, Mishneh Torah, Hilkhot Yesodei HaTorah 7:3)

Not only does the prophet receive a message from God, but he is also given the specific words he should use. During prophecy, the prophet’s free choice is removed; he is merely a channel to pass on the prophecy.

During prophecy, the prophet does not have any free choice at all – neither with regard to its contents nor its extent. He is merely a vessel to pass it on, like a glass mirror that reflects whatever sunlight shines on it. The spirit of God comes with specific words and the order with which he should convey them. The prophet does not choose words to express the meaning he received, as David said: “The spirit of the Lord spoke through me, and His word is on my tongue” (II Samuel 23:2), i.e., that statement came to him through the spirit of God, may He be blessed, and was not merely the intended message with a mandate to improvise words that express the message. Rather, the words themselves were also placed on his tongue by God. He had no choice in the matter; he was merely a vessel: The light of prophecy is seen through him and through him divine speech is heard. (Rabbeinu Nissim of Gerona, Derashot HaRan 5)

In some prophecies the prophet receives only the content of his prophecy, and he must himself choose the words with which to communicate his message. There are also prophecies in which the prophet is also given the specific words to say to the people. Sometimes the prophet is also commanded to do a prophetic act, which arouses higher powers connected with the prophecy.

There are two aspects of prophecy: its content and the specific words [by which the prophet communicates the prophecy]. There are instances where the prophet grasps the content of the prophecy but is not constrained with regard to the words he uses, and can choose the words himself as he wishes. There are other instances, like the prophecies of Isaiah, Jeremiah, and the rest of the prophets whose prophecies are written for subsequent generations, where prophets are also constrained with regard to which words they can use. [The words were chosen] to incorporate multiple meanings. Even in this case, different prophets have different styles, in accordance with their character, their natural language, and style of speech. There are many times when prophets are asked to perform demonstrative acts along with their prophecy, as with Jeremiah’s belt, Ezekiel’s brick, and many similar cases. The point was to arouse, through those actions, whatever higher powers were needed in those matters the prophecy was about, in all its aspects. These powers had been made available and were appointed by God to be manifest at the appropriate time. (Rabbi Moshe Ĥayyim Luzzatto, Derekh Hashem 3:4:8)