The first moments of prophecy were apparently quite capsizing and riotous. As Ramchal puts it, “when a prophet would begin to prophesize he would first experience a great trembling”, as “all his limbs would quiver and quake”. He then begins to “lose consciousness” and enter into a trance.
Understand of course that the prophet wouldn’t be apoplectic or suddenly suffering a seizure, though that must be what he seemed to be going through to the observing eye. He wasn’t ill or crazed either. What was happening was that a part of his personhood was being undone by the presence of G-d and shifting about, and was being replaced by what we could only term a full-bodied, supernatural personal receptivity to whatever G-d would be infusing him with.
He would then begin to experience the sorts of visions that only true prophets would merit seeing, which Ramchal terms “revelations of (G-d’s) Glory”. And the prophet would then understand that he was indeed being made privy to what “Supernal Wisdom deemed he should know”.
The prophet Isaiah described the following quite unearthly moment that could serve as an example of what happened. “I saw the L-rd sitting on a high and exalted throne,” he said, “and His lower extremity filled the Temple. Seraphim stood above Him, with six wings … each; with two (each one) would cover his face, and with two he would cover his feet, and with two he would fly. Each one called out to the other and said, ‘Holy, Holy, Holy is the L-rd of Hosts; the whole earth is full of His glory!’ The doorposts quaked from the voice of him who called, and the house became filled with smoke. And I said, ‘Woe is me … for my eyes have seen the King, the L-rd of Hosts” (Isaiah 6:1-5).
It goes without saying that not everyone merited such revelations. One had to have managed to “attach himself unto G-d’s presence” to a very great degree, in Ramchal’s words, which of course implies involving oneself in all sorts of intellectual, prayer-centered, and mystical practices, and being of the highest moral caliber.
That would enable a potential prophet to achieve “great spiritual stature” and to then be granted the ability to “perform all sorts of miracles and wonders” depending on his standing, aside from his communications.
Rabbi Yaakov Feldman has translated and commented upon “The Gates of Repentance”, “The Path of the Just”, and “The Duties of the Heart” (Jason Aronson Publishers). His works are available in bookstores and in various locations on the Web.