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 15:1-3
1
Then Yahweh said to me, Though Moses and Samuel stood before me, yet my mind would not be toward this people: cast them out of my sight, and let them go forth.
2
It shall happen, when they tell you, Where shall we go forth? Then you shall tell them, Thus says Yahweh: Such as are for death, to death; and such as are for the sword, to the sword; and such as are for the famine, to the famine; and such as are for captivity, to captivity.
3
I will appoint over them four kinds, says Yahweh: the sword to kill, and the dogs to tear, and the birds of the sky, and the animals of the earth, to devour and to destroy.