Speak
Like "say," the word "speak" refers to thoughts and feelings moving from our more internal spiritual levels to our more external ones -- and ultimately from the Lord, who is in a sense the most internal spiritual level of all. This is generally called "influx" and "perception" in the Writings, meaning they are thoughts and feelings that flow in in a complete way from the Lord, rather than being things we have to think about and figure out. On a number of occasions "speak" and "say" are used together; in these cases "speak" refers more to intellectual instruction in matters of thought and "say" refers more to feelings and affections that flow in directly.
Jeremiah 29:19-21
19
because they have not listened to my words, says Yahweh, with which I sent to them my servants the prophets, rising up early and sending them; but you would not hear, says Yahweh.
20
Hear therefore the word of Yahweh, all you of the captivity, whom I have sent away from Jerusalem to Babylon.
21
Thus says Yahweh of Armies, the God of Israel, concerning Ahab the son of Kolaiah, and concerning Zedekiah the son of Maaseiah, who prophesy a lie to you in my name: Behold, I will deliver them into the hand of Nebuchadnezzar king of Babylon; and he shall kill them before your eyes;
