Jamieson, Fausset and Brown on Jeremiah 36:16

16. afraid, both one and other--Hebrew, "fear-stricken," they turned to one another (compare Ge 42:28). This showed, on their part, hesitancy, and some degree of fear of God, but not enough to make them willing to sacrifice the favor of an earthly king.

We will surely tell the king--not the language of threatening but implying that the matter is of such moment that the king ought to be made acquainted with it, so as to seek some remedy against the divine anger.